You can use the Format button on windows (right mouse click on the drive letter)
- or the msdos command format drive: (ex : format d:)

but if you want to format the os drive/partition you will need to boot on another harddrive or floppy disc/cd/dvd ... or use the windows install disk to reinstall and you can format (or change partitions) ....

format button in right mouse click and format can format the hard drive. but the data may be still recovered.