Hi and welcome.
They may clash as they try and read the same files.
Even if you disabled one, the other Anti-virus may still pick up the installed Anti-virus definitions files of the other Av and report these as a virus.
Best to to use just the one.
Uninstall Avast first then install one that you choose.
D/load new Av to desktop>disconnect from the net>uninstall Avast>install new Av.
They may clash as they try and read the same files.
Even if you disabled one, the other Anti-virus may still pick up the installed Anti-virus definitions files of the other Av and report these as a virus.
Best to to use just the one.
Uninstall Avast first then install one that you choose.
D/load new Av to desktop>disconnect from the net>uninstall Avast>install new Av.