I have a Dell Dimension 9100 running XP Home that is freezing fairly regularly. It boots and performs fine, but after a period of time ranging from a few minutes to 30 minutes it gets sluggish responding to events and then freezes completely. I ran a malware and AV scan and found nothing. Windows is up to date.
I then booted into safe mode to run a malware scan and it kicked a BSOD after about 8 minutes. The next time I went into safe mode it ran fine for more than 30 minutes.
Ran HijackThis which came back clean.
I've tested each stick of RAM individually and it freezes at some point for all. Running an AVG scan I could see the system slow down as it was scanning files and then if I clicked around on various windows after a few clicks the system alarm would start a continuous beep and it would lock up.
Finally, I used a CPU stress test utility to warm up the CPU. Doing this will hang the system within a minute every time.

Take software out of the equation.

Download http://www.ultimatebootcd.com/download.html
install and run the programme on any working PC and follow the instructions to create a live CD.
Boot to this CD on the suspect computer.
This will give you a Clean Windows environment with loads of tools to test your system with.

Good luck and let us know how you get on.

I already tried UBCD, ran all the diagnostics and everything passed.

try dowloading and burning to disk the live knoppex in my signature to rule out software

If everything passed and it didn't freeze in UBCD then the issue is you probably have a software issue.(Although your hard drive may be faulty)

Try disabling some startup software by going to start>run> msconfig & unchecking some of the non-microsoft startup services.

Do as sennetor suggested but click "disable all" in "startup" (Do not go online as you will possibly have no virus detector) If you machine has no problems, try enabling the top half and see if it works....................until you find the problem.

This might not help at all but I ran into a similar situation once and it was caused by the CPU overheating. The heat sync on the CPU was clogged with dust and not allowing the fan to exhaust the hot air. The hotter the CPU would get the slow and more non-responsive the PC would be, until eventually the system alarm would continually sound until I shut the system down.

Depending on the age of your pc and your BIOS settings for CPU temperature, this could be your trouble.

Hope this helps

I agree, it appears to be a CPU heating issue, since I can crash it every time using the CPU stress test.
I thought it might be dust, but I thoroughly cleaned the system and made sure all the heat sink fins were clear.

Have you removed the CPU? If so did you replace the cooling gel?
Do you have the option to display your CPU temperature?
Have you reset the shutdown temp of your CPU too low?
Did you try running with all programs disabled?
Disable them and do a stress test then.
Good luck!

I was going to suspect memory, which can also cause errors and erratic things like reboot. It can if it becomes bad. Drive scanning is fairly easy SMART can report if the drive is in fair or bad health. Virtual memory maybe needing some adjustments, over the years, more and more apps slow things down and I tend to move extra files like photos and video off on flash sticks and USB then move them to a storage drive. The less you have on the OS install, the faster it runs, something to keep in mind