I would try making sure you have the latest wireless updates from Microsoft, they had several issues that were corrected by these updates although I seem to remember the problem being the opposite of yours (would connect to WEP but not WPA) but its worth a shot. I have included a couple of links to Microsoft articles on this issue below...
http://support.microsoft.com/kb/826942
http://support.microsoft.com/kb/893357
http://www.microsoft.com/DOWNLOADS/details.aspx?familyid=662BB74D-E7C1-48D6-95EE-1459234F4483&displaylang=en