yeah definately use Spybot Search & Destroy. Ideally start your computer in Safe Mode and then run the program. (after downloading.. you won't be able to download it in Safe Mode) This ensures none of the nasties are resident in memory waiting to put themselves back in as soon as they're removed. A further precaution I usually do is to kill explorer.exe once Spybot S&D is running.. so:
Download Spybot Search & Destroy.
Install it.
Run it for the first time and download any updates.
Don't bother with the scan.
Restart your computer in Safe Mode (tap the F8 key at the very start of your computer starting up)
Log in under your usual account/name (don't go in as Administrator if you're on XP..)
Start Spybot S&D
CTRL-ALT-DEL, Task-Manager, and right-click explorer.exe in the process list and choose to 'End Process'
Go back to Spybot S&D and do the scan / clean.
After Spybot S&D is done, CTRL-ALT-DEL again, go to File (menu) -> New Task (run..), and type explorer.exe to start explorer back up again and then restart your computer normally. Or just restart your computer from the CTRL-ALT-DEL screen.
HiJackThis is the only tool you really need, but it doesn't actually do the job for you. It's up to you to decide what's needs removing and what has to stay. Probably not best if you're unsure. Spybot S&D will do it for you, and also delete and redundant (though inactive) files that are left behind by the nasties.
You get Spybot Search & Destroy from http://security.kolla.de
Don't fall for any of the fake look-alikes that the search engines turn up.