Yes, you should clean it. Buy a can of air to clean out the interior. Don't use a rag or especially a feather duster or any sort of static dusting tool. You don't want to touch your components if you can help it because static electricity destroys circuits and cloth and dusters carry static charges very easily. Don't even think of using a vacuum cleaner to do it. That's even more dangerous.
How often you should do it depends on how quickly dust builds up. I do mine every two or three months. I blow out the heat sink, motherboard, and GPU with an air dusting canister. That's usually enough. Clean off your fans and any filters on your vents as well. Dust can clog them up and reduce performance.
Defraging helps performance, but it honestly won't extend your PC's life any. It needs to be done, but more to keep data from getting corrupted and to keep performance optimal. Running a registry cleaner on occasion doesn't hurt either, but again, won't extend the life of your PC any.
You should also have Antivirus and antimalware. An antiadware program doesn't hurt either. Viruses, Adware, and Malware can all degrade your computer's performance. They won't destroy your hardware, but they can render your computer unusable eventually causing crashes. Some can even hurt you in other ways, such as stealing personal information such as your identity, credit card and banking information, and really ruin your day.
Get protection. Not having Antivirus if you have an internet connection is like making your computer have unprotected sex with a hundred different people every day. Sooner or later, it's going to get infected with something, probably several somethings. It won't extend the life of your computer components, but you should have it anyway.
How long you should play depends on how hard you're running your PC on any particular game. There is no 'number of hours' that anyone can give you. The easiest way to find out how hard your PC is running is by keeping track of your components temperatures.
The best thing to do is to download a hardware monitor and keep track of the temperatures on your HDD, GPU, and CPU. I use RealTemp and CPUID HWMonitor. CPUID gives more information, but RealTemp has a more simplified interface and is easier to understand. Realtemp is my 'at a glance' monitor, but I use CPUID if I really want to know everything that my PC hardware is doing.
Generally speaking you want your idle temps to be between 50-65. Lower than that is even better, but you should worry if you're hitting 70 when your PC is idle. By 'idle' I mean not doing anything intensive like playing a game.
You want your temps to be around 70-80 for your CPU under load. Your GPU should be around 80-90 or so. Your HDD should be around 50-60. If you're reaching higher temperatures than 80 for the CPU, 90 for the GPU, and 50 for the HDD you're running your PC pretty hard and that will shorten it's lifespan. Again, lower temperatures is even better, but anywhere in these ranges is fine.
Playing games that use more resources is more taxing on your computer. If you're playing games that you only barely meet the requirements for or games that are meant to run on more powerful hardware than you have will run your temperatures up and stress your components more. That will shorten the lifespan of your PC. How long you should play depends on how hard a particular game makes your computer run.
Keeping your PCs temperatures regulated is pretty much the most important thing for a long life for a computer. Try to keep your PC in a cool and dry environment. Moisture is bad for electronics, so don't have an air conditioner blowing right on it or anything. Don't keep it in an enclosed space such as inside a cabinet. The more ventilation it has the better.
Do not leave your case open. It's tempting to do so if you're having heat issues and might seem like a good idea at the time. Honestly, it makes things worse. The components in your computer literally attract dust like a magnet, it also disrupts airflow and can actually make it harder to cool your components.. So leave the cover on the case. It also reduces the risk of bad things happening as the case also serves to protect the components from accidental damage.
Another thing that helps is getting after market cooling. The stock cooling will do fine in many computers, but better cooling means a longer PC life. Consider buying a better heat sink, or buying some extra after market fans. Both are cheap.
You don't need to do much to maintain your computer to be honest. Once you get some decent cooling installed, you only really need to keep it free of dust. Using an air can to blow it out every month or two should be enough. You really don't need to bother with replacing your cooling if your temperatures are stable and low enough. It wouldn't hurt, but you really don't need it if you're in the optimal range or lower.
If you decide to add some cooling I can offer a little general advice.
Always ground yourself before touching anything inside your computer. All you need is a small metal chain that's touching the ground. You can even use a wire coat hanger that is touching your skin. Also, don't even open your case without unplugging the computer first.
Be careful removing the heat sink, don't try to just rip it off. Work it gently from side to side to loosen it up after you separate it from the anchor. How you separate it from the anchor varies, most often you'll just need a screwdriver and/or to undo a couple of latches. You'll probably need to put a new anchor on and remove the old ones after you take off the heat sink as different heat sinks usually use different types of anchor.
If you replace your heat sink, make sure you clean off the thermal paste from your CPU before installing the new one. Don't just slap it on top of the old stuff. Thermal paste is a little like weak glue and you need to be gentle and patient when removing the old heat sink. Just use several cotton swabs and some rubbing alcohol to do it. It will take a while to remove it all, but be patient.
Make sure it's dried before you put on the new heat sink and use thermal paste. It's important to be sure you put fresh thermal paste on your CPU before putting a new heat sink on. You want it to cover the CPU, but not leak out the sides once you put the new heat sink on. Be careful how much you use, too much or too little is bad for your computer.
You don't need liquid cooling. It might be tempting and it's expensive enough that you might think it's the best. Liquid cooling isn't all that great. It's expensive and unless you have a super powerful PC and know what you're doing, the risk outweighs the rewards. Liquid cooling is kind of overkill. It's completely unnecessary on a computer like yours. Just get a decent heat sink and as many fans as your mobo and case will take.
It's really not all that hard. You'll honestly probably only need to get a can of air every couple of months or so and blow the dust out if it with that on occasion. Don't try to blow it out with your breath, that's not good for your computer because your breath is very humid and has lots of stuff in it besides just moisture. It'll work for a quick fix, but I don't recommend it unless you've got no other options and the dust has reached critical levels. Air cans may cause a little moisture to form from condensation, but it will dry quickly and shouldn't cause problems. It's much healthier for your computer than your spit at any rate.
I say this because in a year from now, that half way upgrade is already obsolete and the money wasted on only 1 year of usage.
