If you want to format everything and reinstall your OS, just boot from the OS disk. You'll have the option to format durring the installation.
If you just want to format a single drive or diskette open up your command line (I'm assuming you're on XP...or atleast that's the OS I'm using now). Type:
format [drive] /FS:NTFS
where [drive] is the drive you want to format and the filesystem is NTFS (you don't have to specify a filesystem).