WInXP Home SP2 Freezing / locking up

I currently have the following hardwa
AMD Athlon XP 3200+ (2.19Ghz, 512 MB of RAM)
Chaintech GeForce FX 5900
ASUS N7A8X-E Deluxe Mobo

I recently had a problem with my computer's power supply short circuiting, I
fixed this by upgrading from a 230v to a 480v power supply. Since upgrading
the power supply my computer has had an issue with running properly for more
than 10-120 minutes dependent upon which programs I run and how many of them
I run, i.e. if I run Microsoft's Windows media Player by itself generally I
might be able to get close to 2 hours time out of my computer before it'll
completely lock-up/freeze. I've checked heat and while my computer generally
speaking runs cool (based on how the air coming out of it feels and what it's
tools that came with my computer generally list the internal temperatures as
being). I'm not sure if it is either a virus/deletion of a necessary file or
two within windows that caused this (I've checked the microsoft site and my
WinXP Home SP1 CD and both report all necessary files being present and
uninfected) and I've run Microsoft AntiSpyware, Ad-Aware, and Trend Micro's
PC-cillin 2002 Antivirus and all have shown up clean so I'm currently under
the impression that my RAM may need to finally be upgraded (I'm currently
using 512MB), I may need to replace my HDD, or perhaps my motherboard has
suffered some damage in some form or another.

Any advice would be very useful, if I leave any necessary information out
please ask and I'll supply it as I can.