Today I connected to the internet and my firewall notified me that a trojan horse has been detected.Then avast informed me that my virus database has been updated.
This is what my firewall has written:
trojan horse “IntrusePack 1.27b” detected in C:\Program Files\Alwil Software\Avast4\Setup\avast.setup, process id: 4294286097
Description:
Fail to terminate the process.
Afterrwards I scanned my computer and there was no trojan found.Is this an error? Should I worry ? What should I do?
Among the scan results is also the following:
C:\FILEOOOD.CHK\FILEOOOD unable to scan GZIP archive is corrupted.
Thanks for the help in advance. 
What is your firewall?
avast.setup is what checks to see if there are any VPS and program updates when you connect to the Internet. It is not a trojan and it requires access to the Internet.
Right click on the avast icon and hover over Updating… and select ‘iAVS Update’ avast.setup will now try to gain access, allow it in your firewall.