We shouldn't have to be telling you not to let strangers into your computer, but again and again we see cases where people do.

It never works out well.   Once this happens it can take a matter of seconds to root kit a computer.   Unfortunately cleaning it out is not a fast process.

The analogy I give is to imagine if you let me spend two minutes under the hood of your car with a pair of dykes.   How long would it take your mechinic to figure out and repair the damage I could do?