The registry depends if you are 64 bit or 32 bit but it would be hklm\software\<wow6432node>\microsoft\windows\currentversion\uninstall\<instance>. The productGuid should match what is in the setup.ini on the root of the CD. If they don't then you are trying to install a different product or version(MAS90, MAS200, SQL or 4.4, 4.3, etc ). If it is missing the ProductGuid in the registry just add it and use the ID from the setup.ini (making sure it is the same version and prodcut).