When adware can't trick you into installing it, it often resorts to a secretive invasion. Find out how to defend yourself.

Adware Installation Stealth Tactic 1: Expensive Freebie

• How it works: adware may get installed with so-called free software without any mention of it being included anywhere in the software's license or documentation. Or any mention of the bundled software is buried deep within a click-wrap licensing agreement.

• How to protect yourself: It's become an endlessly repeated cliché, but it's true: only install software from developers you trust. That doesn't mean you can never try any software from a new company. Just familiarize yourself with the developer's reputation before opening wide your hard drive. Search the developer's name on search engines. If a dozen anti-spyware advertisements are listed alongside the search results, that's not a good sign.

• How to fight back: If you've already downloaded the expensive freebie, it's probably too late to simply uninstall it. The bundled adware will likely stick around on your computer long after the software that came with it has been sent to the recycling bin. Instead, you need to use an anti-spyware program, and preferably two to be sure.

Tactic 2. Adware Drive-by