Users who have Avast’s antimalware solution on a PC with Intel microprocessors are reporting Blue Screens of Death (BSODs) when installing the Windows 10 Anniversary Update.
(Update 8/5/2016: Avast has issued a patch, as noted here, which appears to solve the problem.)
Avast users report that when the AU process is kicked off, the PC will eventually crash with a “system thread exception not handled” error. The system then reverts to the previous version of Windows, forcing the user to re-download the update (if they hadn’t previously downloaded it to a USB stick) and begin the update process anew. I have a machine running Avast, and I had the same experience as others reported.
Why this matters: Every new piece of code has the potential to introduce bugs, and the AU is indeed a massive piece of code. The purpose behind the Insider builds is to work out all these bugs before the general public encounters them, but it appears Avast’s engineers missed something. Fortunately, Windows already provides anti-malware protection while Avast sorts things out.
What’s happening, and how to fix it
As the complaints piled up, Avast acknowledged the problem in its support forums. Based on posts by users and Avast, it seems to be some sort of conflict between Avast, the AU, and Intel’s virtualization technology—even, perhaps, if virtualization is disabled. Affected products include the Surface Book and Surface Pro 4 but also, apparently, any tablet or notebook running Avast on top of the affected Intel Core chips.
“...[T]he issue really seems to occur only on the last generation of Intel CPUs,” Petr Chytil, a quality assurance director for Avast, wrote on an Avast support forum. “Previous generations of Intel Core CPUs are fine. Moreover, it did not show up during the testing on the Win 10 preview builds.”
If you do have Avast installed and want to upgrade to the AU in the meantime, you’ll need to uninstall the Avast program. If that fails (as it did for me) Avast has also published a dedicated ”avastclear” application to facilitate the uninstallation process. (You may need to reboot into Safe Mode with Networking to get that to work.) After uninstalling Avast, launch the AU again; it should install normally. Don’t re-install Avast after the update completes, however, as the “system thread exception” error reportedly reappears. The Avast update should fix this, however.
Remember, Windows 10 ships with its own built-in antimalware solution, Windows Defender. Until Avast patches its software, you should probably uninstall Avast, complete the update, then either rely on Windows Defender for a day or two or install another free antimalware program of your choice until Avast sorts out its issues.
Update: Avast has issued a patch which appears to have fixed the BSOD issue.