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Get the latest update of avast! it is now at 4.1.369
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As vojtech said probably Thunderbird and as a 0.6 version this is still very much in developement and doing a search in google (thunderbird unable to create folder) returned a large number of hit (not all relevant), but it give an idea that there have been problems with the 0.6 version. Have you got the latest release of thunderbird?
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Check the Thunderbird website for a forum, it may have something relating to that.
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Avast should not have any effect on the functionality of your email program other than act as a filter of email.
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have you rebooted since this problem occured, somethmes that has resolved a glitch.