Vista is a price increase for Windows. Vista Home Basic, which most desktop computers come with, is inadequate even for most home users - forcing an upgrade. Vista's built-in Upgrade Anytime handles that, with versions costing up to $399 - have your credit card ready. Actually all versions are on the machine but must be "activated".
I question your assertion that "most desktop computers come with" Vista Home Basic. Also, how is inadequate compared to the more expensive versions (not as pretty I'll give you)?
Also, it's called "Anytime Upgrade", and the Anytime Upgrade price from Basic to Ultimate is $139. The more likely upgrade destination from Basic would be Home Premium for $79.