When the computer starts go into the BIOS. This is usually done by pressing a key such as delete when the computer starts up. Look in the settings for boot priority, boot order, something like that. You usually see a list of three with harddrive, floppy, cdrom. Make a note of their current settings in case you need to revert. Put it in this order, floppy first, cdrom, harddrive. Then when your computer starts it will look in that order for a boot record.
Then put in your boot disk. You have not told us what Windows you are using. If you are using XP, simply put in your XP disk and you can boot from that to repair Windows or format or whatever.
How did this problem start?