This question to which I already found the answer is posted here in case of someone else encounters it. I decided to post the Q&A here so that SO has something about this error, since I don't know if it's been here before.
This occured after an update of the .NET Framework. Before the update, everything compiled just fine! After the update, I could compile nothing!
The error message is:
Error 1 - Could not find file 'Microsoft.Windows.CommonLanguageRuntime, Version=2.0.50727.0'.