Ah, boot sector virii, now there's a blast from the past. I'll assume you're running win9x and not NT/2000/XP. Whilst the advice of using fdisk to overwrite the MBR is correct, you still have the problem of creating a clean boot disk. Every floppy disk you put in your drive is going to get infected, so you need to create the boot disk on a clean computer and not on the infected machine. Otherwise, you'll reinfect yourself whilst cleaning the boot sector. Once this is done and you've overwritten the boot sector, go into BIOS and enable boot sector virus protection to prevent reinfection.

The next step is to scan the PC for any remaining traces of the virus in case it's a multipartite virus. The preferred way to do this will be by using the repair floppies that any AV program will create for you, and scanning without booting from the hard disk. Once you've done this and the PC is clean, it's safe to start it normally again. Now, you have to go through every single floppy disk you've inserted into the PC whilst it was running. This is because they will all be infected with the virus due to the way BSV's spread.

A bit of advice to prevent this from happening in the future: Check your floppy drive doesn't have any disks in it when you start your PC. Alternatively, change your boot order to start from the hard disk first. This makes it almost impossible to get a BSV as other devices are only checked if the hard disk is in an unbootable condition.