Adware is one of a class of computer threats called spyware. There are many tools available for finding and eliminating spyware. A lot of these tools are free.
However, I have not found a single tool that can be completely relied on. I therefore always recommend that you have three tools installed. This way almost all spyware will be detected.
Among the best free tools are:
Adaware from Lavasoft
Microsoft AntiSpyware from You know who
Spybot S&D