What would you recommend Toby. Luckily I have not had such a call, but now look forward too it.
There are several variations of the scam.
1. Call to your landline: It's a cold-call, they have no idea if you own a PC or not. There is no reason for any Microsoft-related call. Ever. They don't have your contact details. If anyone calls you and says you have a virus, say you know what they're up to and threaten to sue them/call the police, the calls should stop

2. Browser popups: What Dave had. Nothing malicious, it's really just to trick the most unsavvy users. Closing the browser should be enough. Run a Malwarebytes/Superantispyware after just to make sure.
3. Cryptolocker attacks: All the data on your computer gets password protected and locked, and you generally can't even use your mouse. All you will see on the screen is a warning saying you need to pay X amount to retrieve your data. There's not much you can do apart from pay if you don't have backups, otherwise you can just reinstall your operating system and your PC will be fine.
Turning your PC off if you think you have a virus is a bad idea, as some virus only install/launch properly when you start your PC. Best thing to do is turn off your internet connection (wifi switch or unplug the cable) and run an antivirus. I'd recommend getting a large USB stick or external hard drive for photos or music, even most cloud-based storage is cheap nowadays. No point in spending money unless there's data you really can't afford to lose...


