Question : Problem: Windows XP & Gaming, used to work, now it doesn't.

I used to be able to "game"(several games, Unreal Tournament to Rise of Nations), but recently my computer either gives me the blue screen of death or shuts down the game that I'm playing.  I formatted my HDD & tried again, same thing.  I have AMD Athlon XP 2000+, 512Mb DDR, Antec 430W PS, ATI Radeon 9100 128 MB AGP video card, FIC AN11 Stealth Mobo. It used to tell me in the blue screen that it was dumping the memory or something, but now it says different things. And now, it is even giving me a blue screen & shutting down while surfing online(this is new).  I am using the drivers that were all working before, so what could have changed?  Is there a hardware failure somewhere?

Answer : Problem: Windows XP & Gaming, used to work, now it doesn't.

Could be your fan RPMs have gone down.  Could be that it is getting warmer outside so unless you are way north, your PC would be affected if your AC isnt running.  Dust also can be an issue.  Heat is always an enemy when it comes to running AMD, but we do not know yet if that is the issue here.  
Could be just as callandor said,  a Memory problem, you wont know for sure until a new stick is inserted or if you have multiple sticks, remove one at a time.
I have had that problem with a video card I was having issues with. It wasnt seated properly so whenever I would move cords I would get the same results you are getting.
Good luck, these are always the "fun" problems to fix :)