Help please: Intermittent freezing problem

It started about 6 months ago, my computer would freeze after an hour or so of operation. I figured it was my cpu overheating so i swapped the stock HSF for a thermaltake silent boost, system ran fine for a while after.

Now my system rarely gives me 5 minutes of operation before freezing

symptoms: system freezes ( image on computer stays frozen, mouse doesn't move, NO blue screen of death, NO Hard Drive activity, only way to shut down is to hold the front power button down for 5 seconds)

It may be my ears but there seems to be a distinctive change in the whirr of the fans (CPU fan probably, case fans connected to psu), sounds like the fans slow down or something.

I've had this freeze on me during bootup, and when I was in the BIOS. Does this prove that it's a Hardware problem and not a software problem?

I thought it was the power supply so a swapped for a Enermax 530w monster but the problem still occurs,

I thought it may be a video card problem so I switch my ti4200 for a old rage ati128, problem still occurs.

I've tried using one stick of ram at a time, still freezes, should I try some other ram? what are the chances that both sticks of ram are corrupt?

I consider reformating my harddrive to be my last option because I have some files I want to keep and can't backup cuz it freezes everytime I try.

I think my next move would be to purchase another Asus a7n8x mobo and swap that, but I wonder if my operating system (XP PRO sp2) is locked to my current motherboard or not? if anything could I get an answer to this question

could it be the stock northbridge heatsink thats failing?

I'm torn to just scraping this system for a laptop or fixing this.

Thanks for your time

__________________
AMD Athlon 2500+ Barton
Asus A7N8X motherboard
2 * 512mb Infineon PC2700 @ 333mHz
80Gb Maxtor ata133 hd
MSI GF4 ti4200 128mb

 

reinstall windows.
its a software problem

 

Reinstall the system, Why?

@ celery23 You say it also freezes on you while you are in the BIOS. Then your problem is hardware. Download MEMTEST86+ and create a CD from the ISO, boot your computer from the CD and test your memory. Your CPU or Motherboard may be going bad.

And yes the activation is hardware specific, and if you change out the Mobo, you will be reloading and reactivating Windows.