Mail Client: Mozilla Firefox 0.6 Antivirus Avast 4.0 home edition (free licence on registration type) (no program updates available, all virus definition updates)
  1. Get the latest update of avast! it is now at 4.1.369

  2. 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?

  3. Check the Thunderbird website for a forum, it may have something relating to that.

  4. Avast should not have any effect on the functionality of your email program other than act as a filter of email.

  5. have you rebooted since this problem occured, somethmes that has resolved a glitch.

HTH David