There have been a bunch of problems for Windows XP users after upgrading to Service Pack 3. Many of these issues seem to be related to AMD processors and people have had issues in the past when upgrading to Service Pack 2. There are a couple steps to take to isolate the issue and then resolve the problem.
First while the computer is rebooting continue to tap F8 so that you can choose to stop on error instead of rebooting. Once you have reached the Windows Advanced Options Menu choose the following option so you can obtain information about the error causing the reboot: Disable Automatic Restart on System Failure.
Now when the system reboots it will stop at the error so you can write it down and obtain the details. The error I received was on a home brewed AMD processor computer and looked like the below.
The error message after upgrading to Windows XP Service Pack 3 started with *** STOP 0x0000007e which was a known issue. If your error is the same then boot into safe mode and issue the following command.
Click Start >> Run and enter this: reg add HKLM\System\CurrentControlSet\Services\Intelppm /v Start /d 4 /t REG_DWORD
Before clicking OK make sure it is typed letter for letter so there are not any errors. If you get an error typing this in then you have entered something incorrectly. Once the system takes it then restart Windows normally and everything should boot up fine.
If your error is different than the one above and you have an AMD processor try entering the below into Start >> Run. Make sure you have an AMD processor and not an Intel based processor.
sc config intelppm start=disabled
This should disable your Intel based driver that is more than likely causing the problem.