My computer randomly restarts without warning, just freezes and restarts, any help?

I am almost 100% sure it isn't anything to do with RAM but it mostly occurs when I have a few big programs running (Reason being that my friends computer does it and he only has 64MB of RAM and I have like 400 or so MB of RAM) So that solution may be ruled out. Next bit of info, I don't know if its the... show more I am almost 100% sure it isn't anything to do with RAM but it mostly occurs when I have a few big programs running (Reason being that my friends computer does it and he only has 64MB of RAM and I have like 400 or so MB of RAM) So that solution may be ruled out.

Next bit of info, I don't know if its the temperature because it doesn't seem to be set in stone how hot it can get before restarting (It restarts at different temperatures)

Another bit of info, I don't know what else it could be, but it doesn't "always" do this, it varies from time to time how long it goes without restarting (usually shortly after running MSN, Youtube, or Runescape (Yes I play that from time to time, don't dis on it as that doesn't pertain to the answer))

Ok so now anyone have any solutions? I'm getting pretty desperate...
Update: Ok well I can't take it apart by myself, and yes, my friends computer runs perfectly off of that amount of RAM, weird I know...but it does, and mine doesn't and it has more. Ok so I went to the computer thing and it says: AMD Anthlon(tm) 64 Processor 3200+ 2.04 GHz, 448 MB of RAM Physical Address... show more Ok well I can't take it apart by myself, and yes, my friends computer runs perfectly off of that amount of RAM, weird I know...but it does, and mine doesn't and it has more.

Ok so I went to the computer thing and it says:
AMD Anthlon(tm) 64 Processor
3200+
2.04 GHz, 448 MB of RAM
Physical Address Extension
Update 2: Another thing, it started doing this once I upgraded from widnows 2000 to windows XP
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