system
1
OR should I first uninstall avast and then try an online scanner?
rdsu
2
It’s not necessary to uninstall any AV to try an online virus scanner 
DavidR
3
Whilst it is not necessary to uninstal, you can pause the Standard Shield provider (right click on the avast icon > Pause Provider > Standard Shield) just for the duration of the online scan.
Some say you don’t even need to do that, but two AVs can interact and conflict, if the purpose is to confirm, then it would bebest to temporarily pause the standard shield.