anniesboy68 - can we clarify what update disk you mean?
Are you planning to upgrade from one version of Windows to a newer one (ie: Windows 7)?
If so, you can buy a Windows 7 upgrade disk from most software suppliers.
Or do you mean a disk to reinstall the version of Windows that you're already running?
If so, then provided the original installation was a genuine one, you could borrow anyone's disk that carries the same version of Windows and do your reinstallation from that. Make sure you use your own product key, which should be on a sticker on your computer casing.