My foundations are weaker since I read this in Spyware Terminator forum:
Anti-Vir is by far the best free Anti-Virus on the market. From detection rate to resource use it dominates both Avast and AVG. The simple fact that Anti-vir has an extremely powerful heuristics which neither avast nor (free) AVG have. Avast a couple years ago was the big dog on the block but no longer. The only people who disagree are those who just like to have avast on their computer and do not care about protection statistics. Avast is designed to look like it protects quite allot but in reality it does not do anything special. The web scanner for instance is unneeded as well as the email scanner. It makes no difference whether your email downloads the file or not so being its not allowed to execute. The web scanner just moves all of the"temp files" such as cache and so on. into a folder in your system directory then moves them back where they are designed to go. It adds no protection, just makes it looks like it does more then it really does. again it makes no difference so long as the file is not allowed to execute. Avast has no scheduling system. Anti-Vir has a great scheduling system. I could list every possible category and show you how Anti-Vir bests avast but instead of taking my word for it. Download the newest version then go into options and set it up correctly.
In conclusion, Anti-vir’s real time protection can not be touched by any other free AV on the market.
Since AVAST is not what it should be but still a good AV, can I add extra protection by downloading BitDefender Free on demand scanner?
There is no problem in adding another on-demand only AV as a secondary/back-up check. The only problem exists if you install another resident on-access AV.
My only consideration is when you are going to do an on-demand scan with another security application that you ‘pause’ the standard shield. There are some how say it isn’t required, but doing so has no adverse effect.
The problem arises if say BD unpacks an archive file avast will try to scan the files that are being unpacked and if it detects a file that is infected avast will alert and try to lock that file, now BD also wants to scan that file, etc. To my mind that is potential for conflict which can be avoided by pausing avast’s standard shield, this also avoids double scanning and should speed the time taken for the scan.
Thanks! Sounds ok to me! Best would be to have the Avast detection power of Avira. Avast already is best for its update servers functionality!!! That’s why have been with AVAST for more than 6 years now. But internet moves fast!!! 
Your welcome, detection is only one aspect of an AV, it can have the best detection rates but if it doesn’t suit your needs (P2P, IM, email scanning, flexibility, etc.) and one most often missed support, then it isn’t a good choice for ‘you.’