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.
You can download updates via the MS Catalog site click here. Of course you must know what updates you need by KB numbers in order to search for & download the correct ones.
I usually download & slip-stream most of them into the OS prior to installing using nLite click here (vLite is a similar tool for Vista click here). And yet another similar tool is RVM Integrator click here. Haven't seen anything similar for Win7 so far. G