Vista - System Freezes in between - Pls help

Dear Folks,

I have installed Vista Ultimate on my Home PC (with dual-boot option - XP-32, Vista-32, Vista-64). It runs both 32-bit and 64-bit of Vista. I have been facing the problem of system freeze in between. This happens randomly, and I am unable to trace the cause of the same. The same problem is happening on both 32-bit and 64-bit editions. On the same PC, I have Windows XP, which is running without any problems.

I also tried to upgrade the OS to Vista SP1, but this didn't help me. Now, I am wondering is there any way to troubleshoot this problem and get it rectified?
My System Configuration is :
AMD 64-bit processor
3GB RAM
300GB Hard Disk

I don't get any Blue screen, but the system freezes. No mouse movement/No keyboard movement. I have to reboot my CPU.

Any help on finding the culprit of this problem would be appreciated.

Thanks,
Kamlesh
ForwardSourceID:NT000357BA

you can try looking in your error logs,

Left Click on Start Menu.
Right click on Computer and choose manage.
Click on the event viewer.
You now have loads of options to try and pin down what is happening.
Expand the error tab.
Look for things in the last 24 hours.

Also look in your reliability and performance tab which is found in the same place.