I have had both VS2008 and VS2015 installed on my machine for a while, and VS2015 was installed after VS2008.
Now I notice that whenever I click on a VS2008 solution to open it, it won’t ask me anything but to use VS2015 to open it. I have no idea what this is but the same happens with VS2015. This is only whenever I double click on a solution file to open it. If I right click on the .sln file and give it the version to open with (like VS2008 for example),  it is still opened with VS2015 without asking this question. Then only thing I can open VS2008 .sln file with VS2008, is to open VS2008 manually and drag the VS2008 .sln file into it.
If you are in this situation, two things you need to check the registry entry for the file extension. One is for .vcproj file extension, and the other is for .sln file extension.
Make sure following two registry entry is like this:
1) check .vcproj file extension
Here is the picture:
2) check .sln file extension
Here is the picture:
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