Windows 98 users (Non bootable CDs):

You will need to create bootup floppy disks, to do this go to Control Panel > Add/Remove Programs then the Startup tab.

After you have created it, you will need to boot up the PC with it in.

Select CD-ROM Support from the 3 options.

Wait until the loading has finished then type FORMAT C: <press enter>

It will then ask are you sure, press Y.

Once the format has complete, you will need to restart using CD-ROM Support again.

When it's finished loading type E: <press enter>

Then type SETUP <press enter>

Windows 2000 users (Non bootable CDs):

You will need to create bootup floppy disks, to do this go here to download them.

Once created, reboot when disk 1 in and follow the instructions and from there on you should be able to format.

Windows XP/2003/Longhorn users (Bootable CDs):

Boot up the PC with the Windows CD in, you should get the message "Press any key to boot from CD..." if you don't then you will need to set your BIOS to boot from CD before HDD.

The instructions are pretty basic from there, you can choose Quick Format which saves alot of time.