I think I may have helped someone with this just last night. Check Add/Remove programs for an entry that possibly contains the words 'ad service'.
She had a couple of others, but I reckon it shouldn't be too hard to find the culprit from the list of programs that you know YOU installed.
Then reset the homepage in IE/Tools.
Then run Ad Aware with the latest definitions.
Beware though. Some of this crap when it uninstalls, trys to take you to another location in order to inadvertantly install more nuisance material than you had had previously. Which isn't fun.
Random porn toolbars are notoriously bad for this.