I use them both, in conjunction. Each will occasionally find something the other missed. And both are freeware.
I've got Ad-aware, although not just the freeware parts, on my machine at work, and Spybot on my machine at home. I like Spybot slightly better, it seems less intrusive, and completes faster, on comparable machines. (well maybe not all that comparable. I think the home one is about 2x faster, and has 40% more memory, so perhaps the comparison on speed isn't quite fair. OTOH, the home one has two harddrives, each larger than the one on the work machine. I installed SpyBot right after getting something similar that kept resetting my home page and restricting what search engines the search buttom would bring up. It removed it with no problems.
Neither one is an alternative to the other. I find they compliment each other.
Use them both, as each will catch things the other one dosen't.
Actually this is freeware but they use a sneaky way to get you to use it. you can have it removed by going to their website at www.ieplugin.com an choosing the link to remove it. The only bad part is they insist you tell them why you don't want it after they remove it. I gave them an earfull about sneaking their program on my computer disguised as part of the mmorpg I was playing. I checked with the game company and they did not authorize that download.