How to repair or reinstall windows in safe mode.

Hi,

I have toshiba satellite laptop and anyhow its not working.

I tried many things like reboot with c.d, boo-table flesh drive and also pressed "0" until heard bip noise etc...

I turn on laptop it ask me safe mode, safe camand mode and normal mode. and when I choose normal mode its look like doing some process and suddenly turn off and restart and from beginning like ask safe mode, safe camand mode and normal mode.

I cant understand, whats going on with laptop, So If some one know how to sort it out, Please let me know.

Awaiting valuable reply,

Thanks