As with most programs, this one also might write entries into the registry, which may not be properly removed when uninstalled. When you go to install an older verison, those registry entries are still found, which flag that 'unable to install' notice.
Make sure you uninstall the program, and then look in to the C:\Program Files\ folder for any leftover folder, delete those as well.
make sure there is nothing left in the c:\temp folder.
If you did this, and are still unable to install an older version, then there are indeed entries in the registry keeping you from installing the verion you prefer.
You could manually remove every entry in the registry, but some will still be missed since you would not know what CLSID's where installed if any.
This is why you should begin a registry backup policy, backup registry before installing software, then you can always roll back the reg if you want to change the software install with little fuss.
You might have some success with a registry cleaner, which can remove old entries.
http://www.oldversion.com/talk/index.php?a...159&hl=regclean