Unable to format pc

Question

i am working in pc and it got shutdown automatically. I have installed 2 xp and vista, now when i m trying to restart the pc it got to the window selection page and when i select the xp it restarting and when i select the vista it showing that there is problem to load the winxp in C:windowsystems32.even when i try to load from xp cd the page open till setup configuration and it is getting shutdown. the same problem is with vista.if somehow the setup wil reach the page where we can format the drive for fresh installation then i can able to load the xp but the pc restart before that.it is urgent pls suggest some step