Thanks Bobbye.
In the interim I restarted my computer and my antivirus went nuts. I managed to run MBAM and it found 300 (!) bad things (mainly fake-AV). It deleted them, and things seem better, but I still occasionally get a new tab pop up with google in it for no reason. Just about to run GMER and will report back.
If it's still helpful the exeHelper log says
exeHelper by Raktor
Build 20100414
Run at 15:50:48 on 07/08/10
Now searching...
Checking for numerical processes...
Checking for sysguard processes...
Checking for bad processes...
Checking for bad files...
Checking for bad registry entries...
Resetting filetype association for .exe
Resetting filetype association for .com
Resetting userinit and shell values...
Resetting policies...
--Finished--
Seems clean...