What anti-spyware/malware/adware programs should I keep?

I currently have Avira anti-virus and a Zone alarm firewall while running Firefox as my browser. I have superantispyware, malwarebytes, and ad-aware. Ad-aware rarely picks up anything but cookies and I was thinking of getting rid of it in favor of the other two. I would like to keep two or should i keep all three? I was also wondering if i should get real time protection in spyware blaster but do i need that with firefox? Also, more importantly will any of these programs interfere with each other and bog down my computer?? I recently got a new dell about a month ago with Vista so it has decent specs...4.00 GB memory (RAM i think) with 2.10 GHz processor. Thank you