One of my XP computers is constantly rebooting, does not go into safe mode also. I have had this problem several times with other computers and the solution has always been as under :
1) Boot into recovery console and do "chkdsk c: /r"
2) Boot into UBCD4WIN and restore registry.
This has always worked but this time it does not.
Here is what I did so far
1) Ran chkdsk c: /f option from Recovery console, it showed 4Kb bad sectors at the end. Using UBCD4WIN I then restored registry, did not help
2) Tried chkdsk /r, as well as chkdsk /p options also, doing registry restore each time. Same problem
3) Made a f4ubcd and booted. Used HD Tune Pro trial and it showed a bad sector somewhere around 3.1Gb on C:. So I booted into Hiren's dos utilities and ran something called HDAT2 and it discovered and recovered the bad sector. Checked again and now there was no bad sector shown.
4) Again booted into recovery console and ran chkdsk c: /f and again it showed 4kb bad sector. Restored registry anyway but did not work.
Currently trying chkdsk /r option and then fixboot (had done this too at some point before bad sector repair). Then will restore registry.
Edit : Ok, after doing this too same error. Still rebooting.
I did encounter a similar issue that you're facing. Most probably lies on hardware either RAM or hdd & mine is on RAM defect. Try to reseat the ram or use if you have spare working ram or replace your hdd. Cheers.