SpywareBlaster sets "killbits'' (whatever they are) in the registry to prevent certain known bad websites from opening. It also restricts permissions in IE for some sites, by causing them to be in the "restricted" zone automatically. It's more like a switch than a scanner, just not allowing the browser to display certain sites known to have malware problems (adware/spyware etc). It uses no resource, you download it, update it, apply all protection then close it. Updates occur roughly every week. Well worthwhile.
Avast Home has the same detection/removal ability as the pro version. What is restricted in the free version is the ability to schedule scans from the Avast interface, and script blocking.
Other free versions of AV's don't necessarily have the same detection abilities as their "pro" counterparts.
In answer to your original question 4, well I'd go for Avast , definitely. I've used Avira (free) before. If something happened that I couldn't use Avast any more (touch wood) Avira would be my second choice.
