Recently I installed Windows XP in a VM for some Pinta-related work, and started by installing Visual Studio 2010 and .Net 4. When I then tried to install a couple of necessities like Git Extensions and GTK#, the installers errored out telling me that I did not have .Net 3.5 or greater installed. This surprised me, because last time I checked four was greater than three and a half.
However, it turns out that you can have .Net 4 installed completely independently of .Net 3.5, so .Net 4 does not set the registry keys for .Net 3.5. And since WiX-based installers check those registry keys, they think there is no .Net at all. (WiX is an open source installer framework for Windows, and therefore is very popular for open source projects.) The easy fix is simply to install .Net 3.5 alongside, for a more complicated version you can always try some trickery.