Can you kill it by pressing control alt delete and choosing it for death in the task manager? (this can be a cunt on XP cos there are SO many things running on it...if the thing yer after doesn't show up under "applications" and you have to start digging in the processes list).
If so kill it.
Either way....next press the windows key and R, type "msconfig" got to it's startup tab......sometimes the list you will see is alphabetical, sometimes it's by date added, hopefully yours will be by date so recent things will be at the bottom.
deselect anything that looks like it might be the program you're after, this will stop it autorunning on reboot. be brutal, if you off a coupla of legitimate programs too, you can always re-enable them later, nothing you do here is unfixable.
The above should bind and gag the cunt.
Now get adaware (DIRECT FROM LAVASOFT, trust no other source) and "spybot search and destroy".
Let them have a run through your sysyem.
Spark up any real anti virus you have and let it do it's thing, a full scan of at least C:, but preferably the whole machine).
Useful things to have around....
taskinfo2000
and/or security task manager
Can't remember if both are free but I know one of them is.
These both show a list of processes, who started them, how they started them, and a load of other useful stuff....like whether or not it's known as dodgy (graphics drivers alwaays ended up flagged as "evil" BTW, just ignore that)
EDIT: see you did do a sweep....hmmmmmm......I'd still recommend those particular tools (or kaspersky's stuff......but trust fuck all from norton and ESPECIALLY MACAFFEE....I watched that cunt stare right through rampaging viri before never mind sneakware).
And ALWAYS update them prior to scanning.
If none of the above catches it you may be in deep shit, as it may actually be a true virus that's attached itself to a legitimate file.