If you have nothing of importance on your hard drive i feel the easiest and most thorough way to do this to make sure it doesn't happen again is to insert your windows installation cd and reboot the pc. Upon rebooting if you see "press any key to boot from cd" than do so. If you dont see this and the pc trys to boot to the OS that is not working you then have to restart and go into the BIOS "documentation that came with your pc should tell you how to do this". Then you need to change the boot sequence from (1st device:HDD 2nd device: cd rom) TO (1st device: cd rom 2nd device: hdd) now restart and it should give you the option to boot from cd. Do this. It should now tell you that you need to reformat your disk do so. It will then tell you to partition the disk do so. It should then start to install a fresh copy of windows.