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No anti-virus program, anti-malware program, etc can possiblily catch every such malicious crap. Sometimes, it is our own fault for ignoring warnings, opening email attachments, using P2P programs, etc. There are way too many reasons to list why a certain type of program fails to catch everything. In short, nothing nor no one is perfect. That is a true fact of life.
Avast specializes in virus detection. Ewido specializes in trojans and worms. There is a fine line between what is a virus, a trojan, a worm, etc.
your computer has an email worm and/or virus.
Since avast did not catch it (you gave no indication of warnings from Avast about a virus but only that it could not start correctly), it appears you have/had a worm that used the filter.zap file (btw, not a true ZA file but part of the worm) and you use XP as the OS. This is why I suggested Ewido.
We have not heard back from you yet, but, hopefully the worm is now gone.
Our good friend, Tech, has posted instructions on fixing Avast. You may also need to uninstall ZA and re-install it also.
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