I'm getting a System.InvalidProgramException for almost every test in my test assembly when I run PartCover.  When I run MSTest against the test assembly everything works as expected.  It would seem that something is going wrong when the assembly is being instrumented.  MSDN notes that the System.InvalidProgramException exception is:

The exception that is thrown when a program contains invalid Microsoft intermediate language (MSIL) or metadata. This usually indicates an error in the compiler that generated the program.

When I first downloaded and installed PartCover the tests ran fine a few times and then they started failing with the aforementioned exception.  Since then I had to restore my workstation and had to install PartCover again.  The same thing happened, it worked as expected a few times and then started failing with the same exception.