Thanks for all the replies. Unfortunately, I deleted the suspect files from the Vault already!! No doubt, I'll pick up more sometime, and I'll follow the advice above.
1) I was rather surprised to find that a free version would be less effective than a paying one- what's the point in putting one's reliance on an AV programme, if it ain't as near as dammit 100%?
2) Does AVG Free also scan in real-time, other than scheduled scanning?