TCS thanks for that tip, it gave me the hint I needed for a succesfull google search!
Based on that I found and removed 'adservernow.exe' from the System32 folder (which I had done before but it had returned somehow), removed some associated registry keys and uninstalled the program 'Switch' from the Software list in the Control Panel. Initially this seems to have sorted the issue but I'll know for sure next week.
The Symantec page was quite helpful but the computers here all have the McAfee suite installed, and a search on their website didn't turn up anything. I guess that there's a fine line between spyware and viruses and this particular item is not seen as a virus by McAfee. Also it didn't turn up in scans by Ad-aware, Spybot or Spysweeper.
E-liam let me know if I missed anything in those logs! Thanks for your time!