I just had a terrible morning of PC horrors.
I still have XP Pro SP3 (using the updates for 2009 POS to keep it secure - there's a hack for that). I've had Avast Free and Malwarebytes Free (passive) forever. Today I updated my Avast Free and had a very bad meltdown - could not reboot.
After struggling for a while, I rebooted (for the fifth time) into safe mode, checked Event Viewer, got the idea that BOTH Avast and Malwarebytes were having boot issues, went into msconfig, unchecked all Avast and Malwarebyte services and startup entries, was finally able to reboot into full XP successfully (Hallelujah), UNinstalled both Avast and Malwarebytes, rebooted again, reinstalled Avast, rebooted again, tried to reinstall Malwarebytes and immediately had issues. So I UNinstalled Malwareytes again and it's still off.
Avast, please call Malwareybytes and work out the issue, and then PLEASE LET US KNOW.Thanks.