You can have as many on-demand scanners (like the free version of MBAM) as you want. Some of them run a background service, but in general terms, except for when one of them actually is scanning, there is next to no resource usage.
Pretty good idea to have a couple of extra scanners on board, just in case you get something the AV doesn't deal with. And all the AV's can sometimes let something through. (New malware, or some kind of worm that disables the AV etc.)
At one time, while using AVG, it alerted me to a trojan but could not stop the trojan installing its cargo. Don't know why. This hasn't happened to me while using Avast, yet.
If you do get something nasty, you'll be glad you already have the demand scanners installed.
As stated, WD is not a firewall. Vista (like XP) has a built in firewall. There is an application I've read about that sounds pretty good, called
Vista Firewall Control that is basically a user friendly GUI for the built in firewall, that gives you easier to manage control over anything attempting an outbound connection. To know what should be allowed to connect etc takes a bit of study, but it's not a bad idea. If an as-yet unknown trojan slips into your system, the firewall can be thought of as the last opportunity to prevent it phoning home, gathering reinforcements.
This isn't always bulletproof. I believe there are some types of malware that can inject themselves into a legitimate process, and the change isn't always recognized by the firewall, or if it prompts, by the user.
Avast Home has the same detection and cleaning ability as its paid for version. Includes antispyware (part of the AV engine, so it isn't a separate component), and an antirootkit scan that by default runs 8 minutes after startup. I also use Threatfire, a behaviour blocker, a 3rd party firewall (cause the Windows one doesn't control outbound in XP), a thingy called Secunia PSI, which monitors many many programs on the PC and compares the versions against its own database. Basically it makes sure you're patched. Windows and other programs. It's
here.
Scanners etc are only of use to clean an infected system. A bit of prevention is wise. Set scripts in all (except, perhaps, trusted) zones of the browser to "prompt" or "disable". Better still, use a browser like Firefox, with the Noscript add-on installed, and you can see exactly what scripts are attempting to run on any web page.
An application like SpywareBlaster, by Javacool is good. Basically blocks known bad sites from connecting. Uses no resource; needs manual updating roughly every week.
I have installed far too many demand scanners and other tools. Doesn't matter; they don't run at start, and occupy a relatively small disk space. I like to play around with these things. It's become a bit of a hobby. I think for the average user, some kind of AV (Avast is great, IMO), a firewall, disabling scripting, and a demand scanner or two is probably quite adequate.
Lastly, with Avast running, the number of times another scanner has actually found a real threat on the PC over the last year, has been zero. Go figure.