Upgrading Windows is nothing like starting out fresh. Upgrading Windows is like building a house on an unstable foundation and then building another house on top of your existing house because it's nicer. You're already sinking because you built your home on a crappy foundation (meaning you didn't do the things you needed to do when you first got or formatted the computer), but then you add the weight of the shinier house on top of that! There are many things you have to do when you first get your computer and/or after you format and if you don't do that (99% of people don't because they don't know how), your foundation is crap. That's why people think computers are crap after a year or two. That is simply NOT TRUE. Most of my computers are around 2 years old, and they're faster than anything you've ever seen.
It's bad enough that you build your home on a crappy foundation, but then when you add the weight of the nice shinier home (in this case, Windows 7), you just sink faster until your head is buried in the sand. I'm not talking about something that doesn't really matter here. Upgraded computers are VERY likely to have severe registry problems and to be gunked up beyond repair (without a format that is) than non-upgraded computers. You do not want to upgrade unless absolutely necessary.
If you accidentally bought an upgrade disk, download the OEM version of Vista from my server and do a clean install with it. Sure you won't have a license to Vista, but that doesn't matter because you can "test" it for 30 days. Just "upgrade" to Windows 7 IMMEDIATELY afterwards and you'll have a computer that's *almost as good as a freshly wiped one.