Sadly even that isn't a ironclad guarantee. You're clearly serious about protecting your computer, though, and that alone puts you ahead of the pack.

Linux is safer in part because it isn't subject to the same level of attack as is Windows (nowhere near, in fact), but unless you're willing to learn a new operating system and software, it's tough to seriously recommend.

Frank mentioned Webroot software, which I can't believe I forgot about. I used their SpySweeper for a year and liked it very much. Despite the name it was also a powerful antivirus tool. If I was going to return to a paid solution, they'd handily make it to the shortlist.