Desktop

I had this problem at work on a PC after applying XP's SP2 - endless rebooting. Tried installing SP2 thrice before working out it wasn't me. The causes for this can be different, in my case I had to uninstall SP2 following directions at http://support.microsoft.com/default.aspx?kbid=875350 and then check a BIOS microcode version - this site tells more
http://cquirke.mvps.org/sp2intel.htm
But as you say you have updated BIOS I am not sure if this is the fix for you, but worth checking out still.

It's worth going into BIOS and checking the Duty Cycle of the CPU if the Temp is set too low this will cause the problem that you are describing, but other wise I would default to the Ultimate Boot CD available at

http://www.ultimatebootcd.com/

and test the entire system so you can isolate the problem. Because AMD do not make the Chip sets for the M'Boards there can be some really strange things happen with them.

Col

doesn't xp sp2 now have a Start up menu option of 'don't restart system automatically on error'
you need to use that so the system will freeze at the error instead of rebooting.
turn off the pc, then turn on and tap the f8 key while it boots up. you should see the Start up menu options. you could try safe mode also

Try to get into safe mode during startup by hitting F8 during the startup sequence. If you do get into safe mode, use system restore to rollback the changes that were made if you have system restore enabled. Reboot. If your system comes back stable, apply the updates one by one and reboot after each. You will then find out which one is causing the restart cycle and not apply that one once you have fixed up the problem again.