Well first this is not a scam of avast!, but a telephone scam, that is linked only in the fact they are using the avast product to lend a level of credibility as it is a very popular anti-virus. Essentially sine they have a free version there should have been no need to buy anything.
The telephone numbers are generally random diallers or have been bought as a list from disreputable sources.
There was such a telephone scam where they were purporting to be Microsoft representatives and the same bull about "high number of error messages being sent from my computer, malware, <insert own doom scenario>, etc."
The thing is how the hell would they know your address/telephone number based on your computer IP/MAC address, the answer is that they can't and that really is where the alarm bells should start to ring and not when they are telling you how to transfer money using western union.
Well you have taken the right step in reporting it, I don't know if your local Police have a fraud service as this really is what it is. Though I don't think this would fall into the Internet Fraud remit as it was a telephone call.