Thanks for your replies.
Re the security side, I run AVG, Ad-aware and Spybot and keep them up to date (or at least I do when it allows me onto the net
). I do also on occasion do a Panda check and all appears fine. I will check that I'm using Spybot properly but I'd be very surprised if it's a security issue simply because this computer had never been linked to the net until very recently when I set up the network - and the problem I'm encountering has occured from that point, although of course it's always possible that something slipped in real quick. But as I say, it's not showing up in any checks. I also use Zone Alarm firewall.
Re the possibility of the motherboard going, I will do as you suggest and switch to a different slot - but again I'd be surprised if it is this as the connection it shows is so good, plus I have used the PC for a lot of sound work (Cubase, Soundforge etc) and have had no problems there, nor with any other aspect of software or hardware.