So I installed version 1.0.0.303, and renamed it. But I still get the "Do you want to uninstall" message when I try to install version 1.2.404. Help?

Ahh, yes... Once you installed the original CS, you renamed the Construction Set to something else. The thing is, that the uninstall window comes anyways It uninstalls all files regarding the original CS except for the renamed .exe. Then you just install the 1.2.404 back and both will work, the renamed original and the v1.2.

I got it working. Cool.

Another way is to rename the InstallShield-Folder "{23D683DD-93C6-48E6-B84E-78B57778F126}" to something else. If the installer can't find this folder, it will install without uninstall(and without that message).