Computer Keeps Rebooting

What type of system do you have?
The system configuration is important in determining what if the APM is causing you the problem,also the possiblity the power supply is causing reboot
Is there a possiblity that your system may be overheating per the cpu
Check the memory modules to insure they are seated properly.

first , set bios to boot to cd and do repair from xp cd.
second, boot from me start disk and see if you can "recover" to previous date.
third , do a "command Promt" start up and see what error messages pop up.
forth, as soonas you are in copy everything to cd. do a full format and reinstallation of OS.
Hope this helps.

If the PC is not starting even in safe mode I doubt there is a problem with your Hard disk, hope you get the startup screen and then it goes off, I had a similer problem with some of our HP Vectra 400 PCs here and we found it is a hard disk problem,most of the situation there is some bad sectors in windows folder, you will not lose any of your data, but you have to copy the data to anather PC by connecting this HD as secondary and format it after copying the data or you have to get a replacement for it from the supplier.

If the CPU does not maintain a regulated voltage it will reboot. As coincidental as this was problem was to the O/S upgrades, there may be something really hardware related. Check the power capacitors that are in close proximity to the CPU and verify that those components are not leaking electrolyte either at the top or bottom of the cap.