Most computers will come up at the boot time with something like "hit F2" to enter the setup.
You then hit the F2 key and go into the setup. Tell it to seek the floppy first, CD second then the hard drive. This way when you stick a formatted floppy in the drive that has been loaded with the system files, it will boot off of the floppy. As well you need the format and fdisk files.
Then if there is no floppy, it looks for a CD in the drive. If you have a "restore" disk from the maker, it will boot off of the CD and format then restore the system back to what it was new.
This is not black magic but not for the basic computer person either. Personally if you are not fairly experienced I would go with the factory restore CD and let it do everything. It also loads drivers and such which can take someone not experienced forever.