1. avast! relies heavily on VRDB - without it, it can heal only macroviruses and the viruses supported by the intergrated Virus Cleaner (which are the most common ones).
The obvious disadvantage is that you must have "clean" records of your files in the VRDB - i.e. you should install avast and build VRDB on a clean computer. The advantage, however, is (appart from the fact that you can heal even unknown infections) that the executable file will be turned exactly into its original state. Other repair methods may remove the virus code, but they won't be able to restore the file header into the original state - since the information is simply lost. For example, try to fix notepad.exe infected by Elkern/Klez by the usual tools and then start 2 windows of the disinfected exe - I believe they will not work, since the header has been corrupted.
The information written to the VRDB are the most important parts of the EXE file... I think it's something like 1kB per (exe) file.
2. I believe even the other AV programs do that - only they don't tell you. If you want to scan POP/SMTP traffic, you have to route the traffic to the AV (unless the AV is a firewall simultatenously, maybe).
3. It works on both cases
I believe "Normal" should be enough...
4. The script blocker scans scripts executed by your browser on HTML pages. Generally, these scripts should be run in a "safe environment" and should not be allowed to get outside and infect your computer. However, some browsers (you know which ones I mean
contain bugs - and some viruses exploit them - such as VBS:RedLog for example. If you have an older version of IE without the necessary patches, viewing an infected HTML page will infected your computer. With the Script Blocker, the infection is avoided.