After you uninstalled it did you delete the Program Files for it? Then run something like CCleaner to try and get rid of any registry values it might have left, then restart and try installing it again.
You're only eligible if you buy your system after July 1, 2009 though, and your OEM has to have allowed the upgrade as well.
http://www.downloadsquad.com/2009/01/07/vista-to-windows-7-upgrade-date-and-details-revealed/
The first time I tried clicking on it, it went to advanced search fine, but now it just clicks on the adds every time I try :S Maybe theres a certain place you can click and it won't matter, I dunno.