First i'll start off with windows 10 is in preview, Microsoft indicates tat you should not be using it on a production system for these very reasons.
"Remember, trying out an early build like this can be risky. That's why we recommend that you don't install the preview on your primary home or business PC. Unexpected PC crashes could damage or even delete your files, so you should back up everything."
Second, anyone who is used to testing for microsoft knows that is incompatible with antimalware programs and usualy printer dirvers until tose vendors update ther software. They typically release their beta around the same time Microsoft does; but i have yet to see one for avast for windows 10.
Third, and finally the good news! Someone mentioned "Microsoft Security Essentials", In windows 8 and newer the virus engine for MSSE was integrated into windows defender. There is no need to install anything, your already protected. MD defender is the only antimalwere that works with these early builds of windows and should be sufficient enough given you're installing any and all security updates.