Could you please start regedit, go to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager, select the value BootExecute and export it to a file? If you could post the file as an attachment here, I would like to check what could be wrong there.
In XP, if you press F8 when starting Windows and select 'Last known good configuration' from the boot menu, *maybe* the reboot loop is removed...