winxp reboots for no reason

> I have a strange problem it has happen to me a couple of
> times but keeps getting worse all the time, when I reboot
> my PC and it finally starts & shows the log on screen,
> when I'm typing my password, reboots and when the PC
> finally comes up again, and the log on screen apears, and
> I type my password, when it's loading the personal
> settings, the starup music hears, it reboots again, and
> for the third time when I log-on it finally comes up,
> please help, thanks.
>
>
> Jesus.