Here's some advice:
Cut down on the amount of programs running at the same time.
Use one web browser, not two.
Download and install CCleaner from Piriform
(Don't use registry cleaner)
Download and install Defraggler from Piriform
OR Use the default defrag program
Replace Nero w/ IMGBurn or CDBurnerXP
This is what your average 4GB DDR3 system will look like at startup 30% memory usage is a bit high, but I have Avast, Comodo, Everest and a host of services for Logitech and Creative. My memory rarely spikes above 50% even if I'm using multiple programs at the same time. The only time it spikes huge is if I'm watching videos or playing video games.
Your biggest memory hogs atm are WIndows Live Messenger & Firefox. If you have a ton of firefox windows open, that makes sense, otherwise try to cut down on the amount of add-ons you use. Also, see if using the Trillian program will cut down on your memory usage via Windows Live. http://www.trillian.im/
Also, be sure to run regular defrags and anti-virus/malware scans. You should also consider a fresh install. When you do this fresh install, download a program called Macrium Reflect to make a system image & boot disk of your computer. Make this system image right after you install all drivers/required programs. If you make a system image, you can revert to it any time you have problems with windows or want a fresh start. It takes me about 40 minutes to revert to my system image, but thats just because it's large and I use a usb hub that slows it down a bit.
If you go the Macrium Route, make sure that you test the Boot Disk after you make the system image. You'll need to be sure that you can not only boot and control the boot disk but also that you can access your system image on whatever external drive you use to store it.
The last piece of advice I'll give is for you to do hardware diagnostic tests. Even if you have a newer system, you can get bad parts. If possible, download something like the Ultimate Boot CD or Memtest86 and run tests on your memory & hard drive. When you go to the download page, choose an ISO and then download a program like IMGBurn to burn it to a CD. http://www.ultimatebootcd.com