Caller purports to be from Microsoft Technical Support claiming that your Windows computer is infected and sending error messages to their central server.  I don't have a Windows machine at home, but as a computer software developer for Mac and Windows I played along.  He went through a series of steps to try and convince you that your computer is indeed infected (showing you normal Windows operations errors and warning as well as opening a folder from the Windows Run box and appending the word virus to it and telling you that all those files have viruses (it opened the folder with the first term and second did nothing).

I can only imagine that he was planning on selling me a service to clean these things up and all I have to do is hand him my credit card....  

Caller was in a bullpen of other callers and sounded like he was located internationally.

It's a scam - hang up.