Just an update on the computer.

As of yesterday the IRP Hook Rootkit Trojan virus seems to have disappeared from the AVG scans - as others with the same problem found.
Here's hoping it really was just a 'false positive' and isn't lurking hidden on my computer.

This is the first potentially serious threat I've had (only been on broadband since Feb 2014) so I'm probably being a bit paranoid so thanks everyone for helping out. I can't afford to continually pay someone else to disinfect my computer - and a stuffed computer means no NT - now that really would be serious.