In most Higher Education establishments, they have Multiple Volume Licences from Microsoft.
This means that a College can sell a copy of XP or Office pro for £45-£55 each. They all use the same key and, all get logged as auhorised and legal. The same number can be used for 3-400 computers no problem at all.
One would presume that most companies building computers take advantage of a similar scheme. Not sure if there are lower limits on companies using MVL though.
If you dont have a disk or CoA for XP/Office then look for Keyfinder utility and it will tell you the number embedded in the registry.
As a backup obviously!