Their goal is to get you to download software that will take control of you PC/Laptop, under the guise of helping you to rid yourself of a virus. They ask you to look in the PC's Error logs and then try and frighten you by telling you all the errors you see there, show that you have a virus.
Any computer will show errors in the log. These people are scammers and theives. Microsift don't make cold calls, so just hang up. You can't even get Microsoft to help you fix Windows even if YOU call THEM.
If you are a Talktalk customer, there's a service that blocks the last called number. You can block up to ten numbers. Not sure if other phone suppliers offer a similar service.
Fetere sum, ego sum Ergo