XP Rebooting

Check the system for any hardware conflicts or corrupt drivers which is a error feature of XP

If you have time before the reboot turn off the automatic restart feture in the My Computer properties (right-click My Comp icon and select properties, then Advanced tab, Startup and Recovery settings). This will cause the system to display the error the next time it happens (commonly refered to as the 'blue screen of death'). From the error you may get a better idea what is causing the problem. As the previos answer given suggests it is probably a hardware driver issue. (Install anything recently?)

you may find the cmd in the registry

at startup rundll32.exe to shutdown @ startup

in 9x systems the cmd set in dosprmpt is

rundll32.exe user.exe,exitwindows @startup

so im assuming if you go to

start
run
type in MSCONFIG
and click on the startup tab
and disable all
ok
and restart
may fix the problem

if you cant do this because the computer is shutting down try safe mode that will tell you if it enters safe mode that its conflict/virus/software problem or so
via the msconfig utility

however try that and goodluck

p.s. entering safe press f8 after verification of the DMI (Desktop Management Interface) ....

Goodluck