Some of these claim to be phoning from Microsoft.
As cygnusx-1 posted, tell them you are using a Linux operating system. Or confront them directly & tell them that if they know that your system has a virus or any other problems, then they must be hacking your system, & to know this they are contravening the data protection act, & you are going to take action against the call. - - - They will soon hang up
These are common scams. In the first instance, they will claim to get rid of the virus/spyware by asking you to download their removal software, or to remotely access your pc to manually remove it. - - - For this, you will be charged about £80. this will not work because the "virus" has installed itself too deep inside your pc. - - they will then charge you another £70 ish to finish the removal. - - - After you have paid again, your pc will probably be unusable until you pay a local techie to sort it out at a fraction of this cost.
REMEMBER: If anyone phones you to say that your pc has any problems whatsoever - - To know this, they are illegally hacking your pc, or lying. So if they are hacking, they wont phone you.
This scam actually catches a lot of people out & they get stung for a lot of money, so dont forget, nobody that phones you knows whether your pc is infected or not. - - So HANG UP.
Also while I am on this post. If anyone gets any windows that open on their screen to say that their computer has a number of infections & the computer needs to be scanned to remove them. DONT unless it is a message from the antivirus/antispyware program which YOU installed on your pc.
Stay safe
Rob