Let's assume that Compaq paid the license fee to Microsoft, and didn't pirate the software.
But to be on the safe site, ask the vendor to take back the upgrade and sell you the full version if the upgrade refuses to be installed. Any reasonable vendor will do so, if only because he makes more money from the full version.

Btw, are you sure you mean 3000, not 2003? But if you've got the box it doesn't really matter.

Kees