There are some very good, standalone adware scanners that are free. Do some research to find them.
You're probably fine with a good antivirus program, but some of those standalone ones are more comprehensive and have ways to insulate your browser against spyware attacks. Quite often, those offer the best protection you can get and those programs are worth downloading, updating and using to scan your system from time to time.
I went from using only an antivirus scanner for a year to using one in conjuction with an adware/spyware scanner. I was amazed how many files the antivirus scanner missed.