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 clich, 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

How it works: adware may hide in a website's code and download itself automatically onto the site visitor's hard drive. This is often called a "drive-by" installation.

How to protect yourself: drive-by installations of software tend to happen on obscure commercial websites, rather than personal homepages, blogs, or the websites of established businesses. If you can avoid surfing in those kinds of rough waters, you'll be a lot safer from adware attacks.

How to fight back: If you do suspect that a site has downloaded software onto your computer, close it immediately and fire up your anti-spyware and antivirus software. You may also want to delete your browser's cache and also any program downloads folders and temporary internet folders, just in case the adware is a new kind of adware that isn't in your anti-spyware software's database yet.

Tactic 3: The Old-Fashioned Way: Email

How it works: you know the drill: just as with viruses, adware may come as an email attachment. The stealth part is that simply not opening attachments may not be enough to protect you. The attachment may not display an attachment icon and is set to auto-install as soon as the message is opened.

How to protect yourself: make sure your email software does not open attached files automatically. With most new email software applications the option to block automatic downloads of attached files is set as the default. But to be really safe, you should set your anti-spyware software to automatically monitor all email.

How to fight back: delete the offending email without opening it or the attachment (assuming that hasn't happened already). Run a full scan of your hard drive using anti-spyware and antivirus software.