If you have the windows SDK, do a search for "File Functions". This will give you a few ideas.
Also, depends if you really want to do a format or just wipe the disk (delete of all files and directorys).
I think a format is a low level driver function (i.e. its calls the driver for the device (HDD / FDD) and that performs the format. Whereas, operations like delete are handled via the OS.
I am checking a few sources of C Libs that I have to see if there is an easier way of doing this.
If you don't have WinSDK, let me know and I shall try and send the relevant info to you,
Regards
Chris