You will also face a similar problem when you are trying to install a new version of the software when a previous version already exists on the machine for which the installer files are no longer available. [the issue is specific to InstallShield Express]
In that case, you are likely to see the following error - "Another Version of this Product Is Already Installed. Installation of This Version Cannot Continue."
I faced this Windows Installer related problem very recently while trying to install Adobe Captivate 3 over an older version. I could not install Captivate 3 as I had deleted the previous installation files from my desktop.
Tried deleting the entries from registry editor and also the program files folder but nothing worked.
Then tried pointing the old installer to the new setup but that threw another error - "the installed product does not match the installation source(s). Until a matching source is provided or until the installed product and source are synchronized, this action can not be performed."
Finally, the following trick worked - extracted the files to a new folder and ran command at the DOS command prompt for force installation:
MsiExec.exe /I installer.msi REINSTALLMODE=voums REINSTALL=ALL
You will have to replace the installer.msi filename] Works like a charm. To be on the safe side, run the setup.exe again to repair/reinstall the installation.