Dell Express Upgrade to Windows Vista - ???

Several years ago (2007)  I bought a new E521 Desktop From Dell. It was at the exact time when the last Dell machines were being loaded with XP and the next week they had Vista.  The computer I received was loaded with Windows XP Media center Version 2005.. BUT I also purchased the Dell Express Upgrade to Windows Vista package that included the tailored for Dell Vista upgrade disks that arrived about a month after the computer.   Here it is 2011 and now I want to do the upgrade to Vista using the kit and disks supplied to me from Dell.   Following their provided instructions, The first step is to load the Dell Upgrade Assistant Disk and check for compatability.. This test runs and the block "Operating System" shows fail and says "Windows XP is not Windows Vista Compatable"...   What does this mean... This kit (disks) were specifically designed to upgrade this computer and installed Microsoft Windows XP Media Center Edition 2005 to Windows Vista -  Home premium..  Since XP was eventually upgraded to  SP3 could this be the problem??  What should I do?  or should I just let the XP run and forget the Vista Upgrade I bought and paid for ??  I tried to contact Dell buut since the computer was out of warranty.. NO HELP!   Any help, ideas or sugegstions would be appreciated.