This is a common scam James making you think that your computer has been affected by a virus and asking you to install some antivirus software that you have to pay for (giving all your card details etc of course) and you don't need and in addition the software is likely to be malware anyway which would compromise your security even more. Don't go anywhere near it.
I know you said you are running AVG free which is fine but try this website which was recommended to me by my bank when AVG was telling me that I had a trojan horse http://www.f-secure.com/en_UK/ It has a free online scanner to detect and deal with any inections on your computer. I used it and it was very effective, I also trusted it because I knew that my bank wouldn't be recommending anything dodgy.
Dawn