I finally came across the Windows Installer Cleanup Utility. I installed it, removed a final lingering trace of a previous Ad-aware installation, restarted the computer and was able to install Ad-aware with no further problem.
I am having problems installing the latest version of Ad-Aware free. Whenever I try to install it I get the message "An installation package for the product Ad-Aware cannot be found. Try the installation again using a valid copy of the installation package Ad-AwareInstaller.msi" I was unable to uninstall the previous version as it had no uninstall file nor did it appear in Windows add/remove programs list. I eventually removed it manually by deleting files and then ran CCleaner and Registry Mechanic to try to remove any remaining registry entries. I have even done a repair install of Windows (Win 7 Home Premium) but still get the same message... Help!