Hello Ann,
Thank you again for replying.
As you mentioned about the entry in registry editor which is a common entry in almost all registry keys.
It is very unfortunate that none of the suggestions provided in the previous posts helped.
As you have done almost everything to resolve the issue, I would suggest you to perform in-place upgrade on Windows 7 and check if it helps.
Refer this article:
How to Perform an In-Place Upgrade on Windows Vista, Windows 7, Windows Server 2008 & Windows Server 2008 R2
http://support.microsoft.com/kb/2255099
Windows 7 installation disk is required.
Note: I highly recommend backing up all of your important files and folders onto an external storage device before attempting any type of operating system repair or upgrade.
Please keep us informed about the issue after performing in-place upgrade.
Thank you