Installing .NET 1.1 over later versions: Safe?

Solution 1:

Yes, you're good to go. .NET framework versions are designed to install side-by-side. The other versions of the framework will happily continue on with their lives.

One thing I will add is that sometimes newer versions of the framework will patch older versions if necessary.

Solution 2:

The relationships between the .NET Framework versions 2.0, 3.0, and 3.5 differs from the relationships between versions 1.0, 1.1, and 2.0. The .NET Framework 1.0, 1.1, and 2.0 are completely separate from one another, and one version can be present on a computer regardless of whether the other versions are present. When versions 1.0, 1.1, and 2.0 are on the same computer, each version has its own common language runtime, class libraries, compiler, and so forth. Application developers can choose which version to target. For more information, see Side-by-Side Execution, Targeting a Specific .NET Framework Version or Profile, and Using MSBuild to Target Specific Versions of the .NET Framework.

http://msdn.microsoft.com/en-us/library/bb822049.aspx

.NET 4.0 is also a new, isolated version that can run beside any of the others without interferience.