|From what I've read since I posted it seems that there might be more than one possible cause of this. You are right to have scanned for viruses but have not mentioned any malware finders/fixers. Not all nasties get detected by "virus" checkers.|
Here's few things that would be worth a shot while waiting for someone who knows more about this one (apologies if you have already tried these freebies):
CWShredder (single file attending to an explicit exploit which gets a mention).
"Ad-Aware" & "SpyBot Search & Destroy" (both look for malware and sometimes one finds what the other will miss).
I rather like this trojan finder/fixer:
A2FREE - JUST DOWN PAGE
Make sure you get updates to these before running them (select highest version number for CWShredder).
If it still persists (or you feel that way inclined) download HijackThis, run it and copy/paste the log into the following:
Use them in the order given because I have found this makes life easier when you go to the second (which lists all running tasks, good or bad). Let HJT remove any nasties after going to the first link, then run it again and go to the second link with your now reduced log.
When you are satisfied that your system is clean then get SpywareBlaster (which keeps them out rather than fixing them) and consider putting all the good entries shown by HJT into the Ignore List. The latter will then only show anything new that arrives which makes life much easier.
One word of warning, check all wonder fixes with the folk on here, unless you are certain they are from a reliable source. Many are bogus (including spyware/malware finders) and can make things even worse.