AdAware
Adaware from Lavasoft does an excellent job of cleaning out any adware you might have.
This free version does not run in the background and keep you protected so you have to run it manually when you suspect you might be infected..
Running it reguarly such as weekly is even better !
The usual clues that you might be infected are..
1. the computer seems to be horribly slow
2. Popups when your not expecting them. Some web pages have legitimate popups.
3. Theres more traffic going out your internet connection than coming in.
When you run the program it will scan the computer like an antivirus program. Ensure you Update it first to ensure you have the latest definitions.
When it is done scanning it will present you with a list of infected objects.
Use your right mouse button to select all objects it has found.
Select delete all. Anything it finds is quite safe to delete.