Even using the best practices, you can get hit with adware and toolbars (wait one while I duck out to Listerine my fingers).
The usual source? Deceptive installers and deceptive download portals. They’ll get you every time. They’ll switch home pages and search engines. They really come pretty close to malware, imo.
Anyway, you might get asked to help someone with multiple toolbars and adware crap on his box.
There are good programs for that: Adware Cleaner, PC Decrapi...