Also download Malwarebytes' Anti-Malware (Google it), but it does not automatically scan your computer. You need to manually scan your PC with this program.
Avira,Avast,and MSE are all very good and free. I've been using MSE for over 14 months and haven't had any problems. It's easy to use and is synced with Windows Update. Also use Malwarebytes for manual scans and SuperAntiSpyware for adware/cookie tracking removal.