How can I compile using Visual C++ 2008 and link against an older version of the C runtime (I want version 7 instead of 9)?
VS is only an IDE. You can use it to build with an older toolset, but it requires some tweaking - you need install two side-by-side VS installation (e.g. VS2008 + vs6) and load with different VSARGS. See this excellent step by step from Bojan Resnik.
Note that in VS2010 this solution is not applicable - it has a new per project settings->general->"platform toolset". If desired, install this and manually update project settings->VC++ directories (or manually change the default settings).