Avast is a protection suite not just an antivirus.. To make it an antivirus you would need to remove the following :
Web shield - that stops you from visiting a malicious/hacked site
Sandbox - that runs unknown programmes in a virtual environment to ensure that it is not malicious
Streaming updates - regular but small updates as new malware is found
Software updater - although this should really be called vulnerability protection, how many zero day infections are due to security flaws in Java, adobe etc.
Remote Assistance - this allows someone you know to assist in resolving your computer problems via the net
Browser Cleanup - to remove rubbish from your browser that redirect or alter your search engine/home page
Boot Scanner - the ability to run a scan prior to windows loading
Then you would have something similar to Avira, but Avast aims for prevention rather than cure and all of these are strings to that bow.
I have actually asked for an additional module to monitor the system Host file, as the latest wave of adware subverts that.
There are very few true viruses nowadays like sality or other file infectors. Most are either data gatherers or click to pay malware (malicious software)
If you look at this chart
http://www.virusbtn.com/vb100/RAP/RAP-quadrant-Aug12-Feb13-1200.jpg you will see that Avast free (the only one tested) compares very favourably with other solutions including paid suites, and is on par with Kaspersky
Modules that are not in use do not consume any system resources I do not use P2P or IM software, yet I have not removed them in my system as the overhead is nil
In my opinion and I am in no way beholden to Avast, this is the best free protection that you can get
All antiviruses both free and paid will not catch all the malware out there as the malware writers are always one step ahead, as before you can detect something you must know what it is
