Just writing to confirm that the pre-release is working great compared to V5.1.864. BOOT time scan now works correctly (little slower than expected compared to my 4 year old Laptop with WIN XP-PRO running Avast as well) and it also doesn't automatically reschedule itself anymore (when it was not working anyway, in the previous version).
Boot into Windows, browsing, malware scan, running application etc, etc is now back to normal.
However, I do recommend a full clean reinstall, which is faster imho. In my case I updated over the previous official release and restarted, but the OS froze/crashed without an error and did not boot normally. Uninstalled in Safemode. Clean install ran without issue.
System: WIN7 64bit (all updates), C2D@3.8, 4 GB RAM, Creative Xi-FI Titanuim (no more audio issues with pre-release), Software Firewall -ZoneAlarm Pro (no issues with Avast), Teatimer running in Paranoid mode (not activated via Start-up though).