Visual Studio 2004 won’t open solution files generated by newer IDE like Visual Studio 2008.
But if you really needed it, you can apply the following trick.
First of all, this is a dirty hack and in some circumstances resulting solution will not be identical to the original one.
So, the idea is that one can open all “sln” and “vcproj” files to change
Version "9.0"
to
Version "7.0"
or set any other lower version.
After that when loading these files into VS 7.1 you will be asked to convert solution and project files to the current format.
It worked when I tried to compile C++ solution of Sofia SIP open source on my old VS 7.1.