Tech Support Scams are getting a lot of news lately, and we have fielded quite a few calls from people with "this is windows, your computer is infected" type calls, or big scarey flashing lights and sirens telling them to "stop and call us!"
Don't call them, if they call hang up. Thats the entire rule you need to remember.
You aren't infected, but you let them in, you will be. Here is a few sites with accurate information if you need more.
Just remember, nobody can tell what is going on with your computer except you.