BEWARE! I have heard of many cases recently where people are receiving phone calls from someone claiming to work for Microsoft and wanting to fix a problem on the computer. DO NOT BELIEVE THESE PEOPLE! First of all, Microsoft will never call you; they charge by the minute for technical support.
I had the opportunity to get on the phone with one of these criminals when they were trying to call a friend and they are trying to gain root access to your machine through a custom remote desktop application. Once they gain access, they will have free reign on your machine, documents, etc.
In the instance when I spoke with them, they tried to convince me there was a problem by having me got to Start > Run > and type eventvwr; which is nothing more than the event viewer but might seem “high tech” to the average user. They had me filter for errors and said the errors were caused by this problem (that they could never tell what the problem was). Then they had me go to Start > Run > and type services.msc (the services panel) and asked if I noticed that some of the services were not running and clamed that they were not running because of this “problem” once again. It is absolutely normal for some services to be stopped on your machine.
They will attempt to impress you and convince you something is wrong. My guess is that they will probably attempt to grab identity information or install some malware they will later attempt to charge you to remove.