i am running WIndows 7. i have disabled its default security program "windows defender" and instead, i have installed Avast Free antirus and spybot search and destroy. also, i have not installed microsoft security essentials.
our avast antivirus always works in the background, so its 24/7 protection (of course it should be able to connect to the internet at least once a day to obtain the latest virus definition updates)
my spybot search and destroy has been installed with a little tweaking - during the installatio process i disabled the Internet Explorer "tea timer" protection. The "tea timer" is a feature of sypbot that works 24/7 and monitors any network activity. if it detects a network activity from a program, it might ask you if you want to allow or deny it. it works like a firewall but is not necessarily a firewall. i have disabled this feature since i think avast and windows 7 firewall would be sufficient for my security needs.
microsoft security essentials is an excellent and free antivirus software that is downloadable from the MS website. it provides good protection as the updates are from microsoft themselves
the basic idea with any antivirus is that its always updated with the latest antivirus patches. with microsoft or avast or spybot or ad-aware, i think these are all reputable companies that give their best to distribute the latest antivirus patches. so you can keep your current set of security software as is. having too many antivirus programs running on the same computer can produce a small chance of conflict -- antivirus A may interfere with the scan of antivirus b. while they would both detect and remove the virus, the last program to attempt killing the already killed virus may give an error that the target does not exist (no big deal, but you get the idea). also take note that any antivirus that runs 24/7 in the background affects system performance. modern computers have very good hardware, so this is not much of a noticeable slowdown.
windows (from xp, vista to 7) has a builtin firewall feature. you would want to have this firewall enabled all the time (unless of course you are using another firewall software)
to check that your windows firewall is working:
Start -- Control Panel -- Windows Firewall -- then click Turn Windows Firewall on or off
this is a screenshot from windows 7
the default firewall settings from windows xp--vista--7 are already sufficient to protect home and office users. any new programs that require internet access will ask for your permission - to allow or deny.