To start with having two resident anti-virus scanners installed is one too many and not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable.
This could be a conflict, but since you give no information on the avast detection, file name and location (or screenshot of the alert window), this is speculation. Avast runs an anti-rootkit scan 8 minutes after boot and it sounds like this is what has alerted, but the information is still required. It could be that it is detecting another security application hidden service/process.
Check the C:\ProgramData\AVAST Software\Avast\log\aswAr.log (this may well be a hidden folder, if you haven't changed your explorer folder options). This file is overwritten if you have rebooted since the alert, making notes at the time of an alert is advisable.
You can really test another resident antivirus whilst having another resident AV installed.