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?
- Do you use a firewall? Which one?
- Do you have any other antivirus installed in your system?
- Any other security programs that could interfere?
- Any useful information into Control Panel > Administrative Tools > Events, specially 'Errors'.