I suggest that you go join the avsim forums, there is a dedicated PMDG MD11 support forum, or go to the PMDG website and fill out a support ticket, the avsim forums will probably be quicker and if they can not help then go to the PMDG website for help.
I use the PMDG MD11 with Win7 all the time and have not had a problem. The only advice I can offer at this time to to uninstall and when you reinstall make sure that UAC is completely off.
If you post at avsim give more details about the error code you are getting, it will help people in giving a response that can help.