We will need more information to be able to help you:

  • Which OS are you using? Is it up to date?
  • What avast! version and VPS file (virus database) number?
  • Which actions have you taken to try solving the problem?
  • Do you use a firewall? Which one?
  • Do you have any other antivirus installed in your system?
  • Any other security programs that could interfere?
  • In Xp/Vista, any useful information into Control Panel > Administrative Tools > Events, specially ‘Errors’.

Check the folder \data\log
Are there any files called unpXXXX there (where XXXX is a random number)?
If so, send them to vlk (at) avast.com
They may contain more information about the problem (maybe a link to this thread).