On our old laptop a friend gave me links to Avast and Malwarebytes for free protection. Both worked great. Then DD dropped said laptop.
On our new laptop, I tried going to those same links and the Avast one caused my internet to not come up, so I deleted it. The Malwarebytes downloaded just fine, but immediately said it was outdated. A few weeks later I finally clicked on the "update here" button. Upadated, and again, immediately is says it's oudated.
The laptop came with Norton but I didn't renew it. But, Groupon Goods has a $25 deal for 15 months of protection.
Ditto AVG; I used it when I had Windows computers and it works great.
Spybot and AdAware are also both helpful for spyware and adware. I didn't keep those running all the time, though; I would just update it and run it every few months.