New Computer Build Not Working Properly

Hi all, I just built a new computer something like 2-3 months ago and am having a number of problems. First problem is when I leave the computer on sometimes (For BitTorrent program) and computer will just shut off and restart itself:? There is nothing on the event logs about this error and I dont know what is causing it.

The second problem is due with my Ethernet port, becasue I am at student accomidation they only give one Ethernet port in the wall and dont let us use routers so I have to keep swithing between by computer and XBOX 360...sometimes when I switch back to my computer it will not connect to the internet, it will try to over and over but wont do it, to fix this I usally have to switch off my computer for a while and just starts to work.

The third problem started today...I left my computer on BitTorrent and it switched off and restarted itself, when I logged on back to Windows 7 it said in system properites that I have 4GB RAM (Which I have) but only 3.25GB is useable on a 64 bit OS:? And its scoring it from 7.4 (full 4GB) to 5.9 and I dont whats causing this Im thinking the motherboard is maby faulty but I rarther get a second thought before contacting the seller. Any help is greatly appreciated, thanks in advanced:)

OS Windows 7 Professional 64 bit
CPU AMD Phenom II X6 1055T
Motherboard Gigabyte GA-890GPA-UD3H
Memory Kingston HyperX Blu 4.0GB Dual-Channel DDR3(Part Number: KHX1600C9AD3B1K2/4G)
Graphics Card MSI ATI Radeon HD 1GB 5770
Hard Drives 360GB SATA Drive - OS
500GB SATA Drive - Games
1TB SATA Drive - Personal Files
PSU 500W OCZ

Oh sorry forgot to mention...not malware, viruses, spyware etc... scanned it using Avast as well as Spybot S&D also have comodo Firewall installed (just for extra security)...clean up with CCleaner, basically system is tip top shape:)

I would boot into bios and set it to default , the run rig and see what happens

If your computer is restarting randomly that could be any number of problems, Driver problem, overheating problem , Hardware failing etc etc. The Ethernet problem is actually probably more the building that you live in or the ISP they use and not you. My ISP is very very similar, if I am running right off the modem and not my router and I unplug my computer and then plug in something else it won't connect until i restart the modem. It is stupid and irritating but unfortunately their is probably nothing you can do about it. I would be hard pressed to blame that on your Ethernet unless you have a failing mobo. As for the ram problem not to sure what to tell you about that....you sure your running 64 bit 7? right click my computer go to properties and it will tell you. That is a classic sign of a 32 bit OS which I am sure you don't have but can't hurt to double check.

Hit the windows key and the pause/break key. Then click advanced system settings on the left. Under the Advanced tab click the settings button under Startup and Recovery. In the System failure section remove the check mark next to Automatically restart. Instead of rebooting you should get a BSOD with a Stop error that you can Google. If its already un-checked you'll have to look at your event logs etc to try and figure it out.

If you add a second NIC to your PC you should be able to bridge your other NIC to it. Its the equivalent to Internet Connection Sharing. Once you do that you can connect your Xbox to your PC and they both will get INTERNET access. Depending on what NIC you add you may or may not have to use a cross over cable between your PC and the Xbox. Gigabyte NICs can auto sense and use a regular network cable, 10/100 BaseT NICs usually can't.

A switch will work if they allow you more than on IP address for your connection. And you won't have to leave your computer on for your Xbox to work. Thats a good suggestion.