Some suggestions. Try them in order listed.
Disable automatic restart on system failure. Just to see if you can get a STOP code, or if you can login to Safe mode, then
it's possible you can restart the system in normal mode later on.
When the pc is booting up (after the BIOS has done its POST test and
before Windows starts loading),
Tap F8 Function key to get bootup options. Tap & keep repeat tapping F8 !
You will actually see "Disable automatic restart on system failure" as an option .
Select that and give it a try. This option is available if you have XP service pack 2.
Once restart on failure is disabled, you may get a STOP error with
text --- at the next Windows startup.
Write it all down, and then start searching for the final solution.
And post back here with the STOP code & text.
B) If no stop code, and still unable to login to Safe mode, reboot pc, repeat the F8 tap, select the option with "VGA mode".
See if you can login.
If still unable to get into any mode......
Last, C) I'd do F-8 Function-key selective bootup and try hard to select Last Known Good.
When the pc is booting up (after the BIOS has done its POST test and
before Windows starts loading),
Tap F8 Function key to get bootup options. Tap & keep repeat tapping F8 !
Once restart on failure is disabled, you may get a STOP error with text --- at the next Windows startup.
Write it all down, and then start searching for the final solution.
See Aumha.org page on STOP MESSAGES
http://aumha.org/win5/kbestop.htm
Best of luck. Do post back here with details.