WIN32 API hooking for .NET applications?

[jonwil]

I have a .NET application that calls some native APIs via dllimport (including APIs in system dlls like user32.dll). I do not have the ability to recompile (or modify) this .NET application. Is there a way I can hook these native APIs somehow? (i.e. intercept the user32 calls, do some stuff and then call the original user32 function)
Some of the calls I want to intercept are being made by the application itself (and dlls that are alongside it) and some are being made by .NET runtime components.