We will need more information to be able to help you:
- Which OS are you using? Is it up to date?
- Which actions have you taken to try solving the problem?
- Do you use a firewall? Which one?
- As David asked, do you have any other antivirus installed in your system? Did you have in the past?
- Any other security programs that could interfere?
- In XP/Vista, is there any useful information into Control Panel > Administrative Tools > Events, specially 'Errors'?