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).