No matter how hard you try or how far you go, you can not ensure a virus free system for ever. There are many Antivirus tools that claim to detect almost all type of Worms and Trojans but still there are chances of them missing some.
One of the solution to the above problem is to have more than one antivirus application running in your system but that’s not at all possible as most of the antivirus programs are not compatible to work with one another and even if you may fine a compatible match I recommend to drop the idea as it will make your system more sluggish and creepy.
Now with AVERT, a freeware antivirus application, you can ensure a secure system with more than 8 antivirus scanners scanning your files at the same time. Although the application lacks real time file scanning but there are three modes of on demand scan that are Complete scan, Balanced scan and Quick scan. AVERT installer is only about 3Mb in size which means after installing you can download your favorite scanner and virus definition from the respective servers.
[You must be registered and logged in to see this link.]Antivirus engines in Avert Virus Scanner

1. Asquared
   
2. Trend Micro
   
3. Sophos
   
4. Avira
   
5. ClamWin
   
6. VIPRE
   
7. Panda
   
8. Vba32
   

Some noteworthy points:

1. As this antivirus does not provide a real time scanner I advice not to rely on it completely, although due to its portable functionality you can always use it as a fail safe.
   
2. There can be many false detection by the program, for instance when I was testing the software Avira detected my Tata modem driver as a Trojan, so please be very sure as to what files you quarantine after a complete scan.
   
3. It includes tools to backup registry, reset Windows HOSTS file and reset Windows proxy to resolve potential internet issues.
   

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