In the end I did manage to upgrade to Windows 7 Professional and it works great but I first had to uninstall Kaspersky Anti-Virus 2010. There was no error related to this but a DRM error and bluetooth error in the C:\$WINDOWS.~BT\Sources\Panther\setuperr.log was mentioned as a tip in a comment on a blog post I read. After that I did an in-place upgrade from Windows 7 Home Premium to Professional. I had done some Windows Updates prior to the upgrade from Home to Pro which was a mistake because I had to do them again
I edited the registry according to Hack to In-Place “Downgrade” from Windows 7 Ultimate or Professional to Less Premium Editions « My Digital Life
to do the upgrade (basically change the edition and version in the registry).
Forgot to add that the DELL OEM key on the PC didn't work for the Anytime Upgrade (which I tried before the in-place one) but the in-place one worked because it needs it during the install (I didn't give it a key on the Vista to Windows 7 Home Premium upgrade at the outset, it was using the original older Vista one). At first it said it wasn't activated but when I checked it and asked to do it again it succeeded. Thanks again for the help!