It doesn't offer to load in safe mode or anything at boot? If it restarts in middle of the boot process, whatever the cause, it should do that, unless it's already made it to the login screen (or somewhere around there)...
Without knowing any other details, all I can really suggests is a combination of FIXBOOT&FIXMBR (tools available on the XP install disc, IDK if the equivalents in Vista/7 go by different names), or if that don't work, repair install.