Try some anti-adware programs, such as the free Ad-Aware and Spybot S&D. They tend to be better at getting rid of adware than many anti-virus programs.
Also, try running them in Safe Mode With Networking:
1. Log out and reboot your machine.
2. When the machine starts the reboot sequence, press the F8 key repeatedly.
3. Select Safe Mode or Safe Mode with Networking from the resulting menu.
4. Login. If the malware has changed your password, try logging in as Administrator. By default, Administrator has no password.
5. The machine will continue booting, but the Windows desktop will look different.
6. When you're finished doing what you need to do, log out and reboot back into normal mode.
Note that even if the anti-malware programs get rid of the malware, they may not be able to reverse the effects. Search the Web for possible fixes.
Update and run everything regularly, not just when you think you already have malware.