Well, Microsoft will not send you a virus, but they can do one of the following:
- Lock out your copy of any MS product that has been installed illegally
- Send out the SPA (Software Piracy Agency) to your door step with Warrant in hand to view your pc and any information stored on it
-Press charges with a very good chance of winning. Typically they can charge a maximum of $250,000 per incident, but they usually (assuming you are an individual) make an example of you and ask for a portion of that amount if found guilty.
I know there are a few others yet I can't find them at the moment. A company I used to work for was fined for 18 Million dollars because of Illegal software they were running instead of purchasing licensed copies.
Getting caught is no picnic.