Decting previous installation of MyApp

I am creating a install script for my application, say MyApp. I would like to check if the user has installed the previous version of MyApp on his computer and then determine whether he/she is going to install the new version of MyApp in the same folder as that of the previous version. If he/she is installing the new version in the same folder as the previous version then I want to remove the old registry key entries and the old program group items.

I don't know how to check if the user is going to install the new version into the same folder as that of the old version.

Once I could figure out the above, I know how to remove the registry keys and the program group for the old installation.

The users may install MyApp on Windows 9x/Me/NT4/2000/XP/2003.

Please advise.

Thanks,