-Edit- I feel like an idiot. I had a feeling something like the answer below would work but didnt see any google results similar to the answers below. So when i saw this com
Use:
System.Diagnostics.Process.Start(filePath);
It will use the default program that would be opened as if you just clicked on it. Admittedly it doesn't let you choose the program that will run... but assuming that you want to mimic the behaviour that would be used if the user were to double-click on the file, this should work just fine.