Hi Karl,
Which program are you trying to install?
What is the full/exact error message that you get when you try to install the program?
Windows 7 comes with 3.5 version of .NET Framework and all the earlier versions of .NET Framework is pre-installed with the same. As you are saying that the program that you are installing is a pre-Windows 7 program, it might not be compatible with Windows 7. I would suggest you to check the compatibility of the program in the below given compatibility link.
http://www.microsoft.com/windows/compatibility/windows-7/en-us/default.aspx
Also, you may try and install the program in compatibility mode and check if that works fine. To install any application in compatibility mode follow the link given below.
Make older programs run in this version of Windows
http://windows.microsoft.com/en-us/windows7/Make-older-programs-run-in-this-version-of-Windows
Hope this information is helpful.
Amrita M
Microsoft Answers Support Engineer
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