Try starting in safemode by holding down the f8 key on startup.
are you running xp?
If so go to the start icon then control panel then system then the advanced tab then start up and recovery box at the bottom click on settings and untick automatically restart.
This should stop repeated rebooting and give a error code telling you what is wrong.
whilst in safemode go to start icon then click on run when box opens type in chkdsk /r this will check hardrive for errors,there is a space after chkdsk then the /r,it might say windows is using it and will check on reboot click on yes then reboot.
Let us know what happens