Hello All,
I used to use Ad-Aware and Spybot SD, and I have used Ewido also. IMHO the best antispyware software available is Webroot SpySweeper. It's not free, but at less than $30,00 for a 2 year subsription, it's "almost" free.
http://www.webroot.com/
I've used this tool for about a year now, and recommend it to anyone who is looking for an antispyware solution. 2 great features are the automatic updates, and the scheduled scan option. I scan my machines every night.
If you want to be safe while surfing, and keep the "nasties" out in the first place, try Sandboxie:
http://www.sandboxie.com/
You can run your browser (or any other program) within a "virtual" environment, separate from your "real" OS. When you are finished surfing, close the browser, right click the Sandboxie icon in the task tray, and choose "Delete Contents" under the "Contents of Sandbox".
Sandboxie is a free program.
I'm not familair with Prevex1, but I agree wholeheartedly with SirKenin regarding Avast. It's my AV of choice, and has been for a while now. I used to use AVG, but switched to Avast 2 years ago. It has a feature that allows "scan on boot", which scans the system for viruses during bootup, before Windows loads. Since Windows is not running, and the viruses are not in memory, they can be removed more effectively.
Hope this helps,
Jimmy