In the past, whenever you purchase an upgrade release of a Microsoft OS, you have to provide proof that you own a previous version for the upgrade to continue. Typically, you prove this by either installing on top of an eligible OS or by inserting the older installation media during the installation process. Well, the same is true for Vista upgrades versions, with the exception that the Windows Vista upgrade counts an unactivated copy of Vista itself as an ‘older’ copy.

Using this trick, you can get a fully functional legal copy of the Vista version of your choice for a significantly cheaper price since you just need the upgrade version. This workaround is apparently here to stay because Microsoft knows about it and still included the trick with SP1.

While I haven’t tried this personally, it sounds like you can really save yourself some cash and go with just an upgrade copy of Windows Vista instead of the full retail version.