Create a startup diskette

In Windows 95/98/ME you can do so from Start/Settings/Control Panel/Add-Remove Programs.

In Add-Remove, click on the StartUp Disk tab, then use the Wizard to create the diskette.
Once the diskette is done, go to the C:\Windows\Command folder.
Copy the format.com file to the diskette.
Reboot the computer from the diskette
At the A prompt type Format C: and press enter.

Don't forget to first back up any data you want to preserve.

If you are reinstalling Windows 98/ME the start up diskette you create with those Windows versions will automatically include the drivers for your CD-ROM. So when you reboot from the diskette to reinstall your programs, choose "With CD-ROM support" from the bootup menu.

If you are reformatting and reinstalling with XP, the installation process will offer you the choice of formatting the hard drive first.

If you created a Startup diskette in Windows 95 you will have to add the CD-ROM drivers yourself and configure the boot diskette to load those drivers. That is usually difficult to do, unless you have a manual for your CD-ROM drive or for your computer.