Not sure exactly how they do it.
I know that one our forum member posted a thread regarding these types of scare tactics and scams recently here
http://windows7forums.com/windows-7...s-scam-offering-computer-help.html#post241670 it contains a link to what has evidently been going on in Canada.
Additionally, Microsoft has their own off line virus and malware scanner which I've found to be a reasonably good product as well. You might want to give it a try also. Just download the program (get the correct version for your architecture, 32 or 64 bit) and a blank CD. Double click the program and it will create a bootable CD for you which you can use to boot your computer and perform an off line
FULL system scan, independent of your Operating System.
What is Windows Defender Offline?