No, it's not a bug. It's just some certain circumstances on your system. Since you haven't mentioned what other security programs you have installed on your system, we can not tell you much. Maybe some of those programs was blocking avast! If it's a bug, everybody would have same problem, but you are just one of few people reported that in here... as I already mentioned - combination of some circumstances on your machine. Every computer and every configuration is different, so no one can predict what will you have on your system when you decide to install avast!
Also, do you have some other antivirus installed on your system parallel with avast! ? You have to be aware that running two or more resident antiviruses on the same time on same machine won't give you any more protection, but some additional troubles, you will experience, and that's for sure.
Another thing... maybe you don't have another antivirus installed any more, but it could be that you still have some traces of your old antivirus. It could be that you didn't uninstall it properly... it could be many things... it could be that your Database storage was set to some value that your computer is not ready to use at this point (XML or ODBC), so changing that value could also help.
One more idea - are you sure your personal account in Windows has Administrator rights or is it just account with limited rights ?
