Force the System's Default .NET Framework to Prior Version

Is there a way to make my programs open by default with .N开发者_开发技巧ET Framework 3.5 instead of 4.0 on Windows 7 (unless the application was specifically targeted at 4.0)?

(Note: I am not asking for project targets; I'm asking for a system-wide switch, independent of the application.)

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I always thought that when I have got both .NET 3.5 and 4.0 installed, then all applications specifically targetted to 3.5 always choose to run on 3.5 without any additional "forces".

edit: OK, I searched in books and found that this is correct.