With the help of two of my tech savvy friends i built a desktop PC a few weeks ago. Here's the hardware involved:

Case: Thermaltake SViking VA4000BWS Black Steel ATX Mid Tower
Motherboard: Asus A8N4X
Hard Drive: WD740 Western Digital Raptor
Graphics Card: XFX PV-T71G-UDE7 GeForce 7900 GT EXTREME (520MHz) 256MB GDDR3 PCI Express x16 Video Card
PSU: Rosewill RP550S-2MK ATX 2.01 550W Modular Power Supply /Black
Processor: AMD Athlon 64 X2 4400+ Toledo 2000MHz HT Socket 939 Dual Core Processor Model ADA4400CDBOX - Retail
RAM: 2 G.SKILL 1GB 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered System Memory Model F1-3200PHU1-1GBNS
Optical drive: NEC 16X DVD±R DVD Burner Beige IDE Model ND-3550A

Windows XP Pro is a legal copy and it installed without a hitch.

The front fan stopped working after a week of trying to get the computer to operate correctly.

I've never gotten the thing to work as it should. Occasionally the system will BSOD when i try to install programs and immediately restart, even after updating all drivers. Explorer.exe will randomly crash on start-up or I'll get a message that says "The system has recovered from a critical error". The last thing i tried was fully updating windows. That was a bad idea: now my computer simply BSODs as soon as the Windows XP loading screen goes away.

I've reinstalled Windows XP somewhere between 6 and 10 times at this point, and i can't afford to waste anymore time blindly trying to find out what the hell is wrong.

Can anyone offer me advice as to what could possibly be wrong, and how i might go about testing it and fixing it?