Acorrding to the document I just read, global Windows compatibility for .NET Framework 3.5 is 10%: link
This is not good news for my project where the installer is built in .NET and my application is C++. What are ways to combat this problem without having to require users to install any additional framework manually?
Use .Net 2.0... there are no other opportunities for you. Additonally you won't need the most 3.5 Features at all if you wrote a c++ Programm...
I'm not sure I agree with your interpretation.
Although only 11% of PC's have .NET 3.5 natively installed (with the OS), 3.5 is freely available and compatable with 99.04% of PC's.
(I'm run .NET 3.5 on a 2002 era Celeron)
If you are using a .net setup project to build an installer msi, then there are settings in the setup project that allow you to specify that the required framework should be downloaded and installed automatically. You can also specify that the framework be included, see http://support.microsoft.com/kb/324733
Your installer probably does not use any of the new stuff in 3.5. 3.5 is just an addon to 2.0. It should work fine.
The .net install experience can be made fairly painless. The Visual Studio installer project can do this for you, or you can roll your own (eg we do with NSIS and make it almost invisible although it does take a few minutes). Here is an example:
I don't know what C++ requires, but there is also the 'client profile' version of .Net 3.5, which is smaller than the full version.