Like many others, I have upgraded my free version from 10.4.2233 to 11.1.2241 on one of two Acer computers both running Win7-64. Almost immediately I started having problems. Firefox 41.0.2 and 42.0.0 starting locking up with "Not responding" messages. The Avast control panel, the Show Desktop button, and most aggravating, Meta Products MailChecker would run a few minutes then lock up. Doing an Avast repair, turning off various features, and adding MailChecker to the exclusion list did not help. I saw some posts that solved similar problems by reverting to earlier versions. In desperation, I uninstalled Avast and now everything is running fine, and much faster. Since the other computer is running 10.4.2233 and slightly slower but not crashing, I'll probably reinstall that version later today. So my question is, how far back in versions can I safely go? I have the full install for the latest four versions.