I have had a multitude of calls from the "Microsoft Windows Help Desk" , Indian call centre and yes, it's a scam but not in the way you might think. From what I've read this is an attempt to sell you some worthless anti virus software, they do this by highlighting any red x's and warning messages in the event viewer system log, anyone who knows anything about PC's will know that no matter how clean your system is these red x's will show up, they are nothing more than simple system errors.
However, this could be a new scam as I just tell them to (insert profanity here) when they phone these days.
Pete