This computer used to have MEP 2011 installed on it and now im trying to reinstall it as a back-up workstation. i can't get it to install because it says it is already installed. It is not listed in the Uninstall program list either.
We have had this happen before on several Autodesk products. I would suggest following the below Autodesk KB. As a last resort google "Microsoft MSI cleanup utility". We sometimes have to run this in order to clean up installs after they have not removed correctly. I will caution you though to only select the Autodesk products from the list when using the tool.
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=2887771&linkID=9240617
Did you try looking for the Microsoft Windows Install cleanup utility?
http://technet.microsoft.com/en-us/magazine/2008.08.utilityspotlight.aspx
Past that you may want to open a ticket with Autodesk Support. I was talking with one of the AMEP support tech's that I know out at Autodesk University and he mentioned some hidden reg key's that they had just found dealing with Windows 7 and the Install engine that they use. I wonder if these are causing your issue.