Used to get these all the time. They then changed to pretending to call from our largest national ISP. I've also had clients that have victims of this type of scam.
The scam goes a little something like this. They will get you to open the event viewer on your computer, and read out any error. They'll then pretend that this is some major issue that requires that they gain access to your PC. They'll then instruct you to download and run a remote package like Teamviewer or GotoAssist.
Once they are in, they configure the remote support software to run automatically at startup, so they can get in any time they like. They'll also likely request some credit card details to fix whatever imaginary problem they find. They may also infect the PC with an actual bit of ransomware, or ferret around looking for personal information, the jackpot being banking details. Remember, once they are in, getting a browser to reveal all your login details for websites is a simple task.
By the way, love the image with the cat !
Ah yes, pretty basic stuff but enough to get rich quick from the small percent of people who do fall for it.
Thanks for filling in the details. Hack-cat salutes you :)