With all the build-up of useless registry files,
.DLL's and a plethora of other problems, especially with Vista, I don't know why anyone would UPGRADE to windows 7.
It has always been recommended to FORMAT and clean install! I think it's ridiculous that Microsoft even sells upgrade disks. Upgrading is not really more convenient if it causes more problems in the long run. Everyone should know that for the best stability and performance, just do a clean install. I understand that they sell upgrade disks because they believe the people who already purchased earlier versions should pay less for their license, but upgrading an OS is stupidity.
If money is the issue, there are many ways around that to get a licensed copy of 7 legally. This article mentions something about student upgrades? Students can get MSDN accounts through academic alliance. I don't see what the problem is. I have never and never will upgrade a Windows platform. Take the extra time to backup your stuff, and JUST CLEAN INSTALL IT!