One is good enough, but...

I've used as many as four- Spybot Search & Destroy, Hijack This, AdAware, and Coolwwwshredder.

Some of the anti-adware programs are not as quick to update their signature lists as others are so if our system becomes infected with something none of the above can detect, we tend to do a search for information regarding the recent infectant and we may come across a support forum that suggests we download this or that program (different than the ones I've mentioned above) in order to have the problem immediately taken care of.

If anyone could wait a few days, AdAware would eventually have their lists updated to include the most recent infectant. If not, it should be brought to their attention so that they can research and develop the cure for that particular infectant.

As for several programs finding adware in each other's quarantine folders, that rarely happens because most programs have a smart scan that avoids other anti-adware program folders. You can see the complete path to each file and rarely do any of these reside in other anti-adware program folders.

Having one anti-adware program (AdAware), an anti-virus program, firewall, knowledge of your system files (& recent modifications), and disconnecting your internet during reboots or whenever the system is not in use is the best way to help combat adware.

For anyone using SP2, you can disable BHO's (browser helper objects) through the setting: tools/internet options/programs/manage add-ons. Always keep IE pop-up blocker enabled. Additionally, you should add known adware websites to your restricted sites zone. (abetterinternet.com, webrebates.com, etc..)