continuous reboot loop

Was there a motherboard change just before the problem started?
If yes, you may have to reinstall Windows.
If no, can you boot to Safe Mode?
If yes, disable all startup programs. If problem stays, do ?System Restore? and do ?chkdsk /r? from Recovery Console if necessary.
If no, do ?chkdsk /r? from Recovery Console.

If the problem persists, you have hardware (HW) problem.

1. The hard drive (HD) may have crashed. Do a HD diagnostic test. Download a utility program from the HD manufacturer?s website. If the test shows HD failure, replace it and clean install Windows. You may be able to access the HD to retrieve data.

2. If the test passes, you may have a bad CPU or motherboard. Take your system to a local service place to troubleshoot.

when did this crop up?
get the xp installation cd. boot from it and tap r until you get to recover console login prompt. login with local administrator account. password maybe be blank or hopefully you know it.
from there, i would first run fixsasser then full anti-virus scan with updated defs. if it stays up thru all that you know its not hardware. if it won't stay up even booting from cd you know it is hardware. anyhow. once you are back at recover console desktop, right click my computer, get properties, pick advanced. down at bottom change settings to not reboot on error. now you will get blue screen. post entire thing and we can look it up. or look up yourself at support.microsoft.com

I had a similiar problem to this last week, but the PC was hit by the Sasser virus. You can try using bootup disk to repair the OS.

I tried it that way initially, and was able to log in and access the desktop, but eventually had to wipe it and do a clean install.

Your Master Boot Record may be infected.