If you're using NT4 or 2000, and you have a system with a BIOS that can be set to boot the CDROM and you have tested this to make sure it works then you're in luck. All you have to do to install / restore the operating system is boot the CDROM. Unless you've got only SCSI hard drives this method will work.

If you can't boot the CDROM you can always make boot diskettes as follows:
For NT4, winnt /ox
For 2000, d:\bootdisk\Makeboot.exe a: (where d: is the letter of the CDROM drive and a: is the floppy)

For Windows 95 or 98, click on Start / Settings / Control Panel. Then click on Add/Remove Programs and select the Startup Disk tab. This creates a startup disk which is bootable and can be used to boot the system to command mode in order to install any version of Windows-9X, even a non-matching one.

Please see this page for details on how to (re)install Windows once you've done this.