I have just troubleshot a rather odd problem, and am posting the details here to alert the devs and help anyone else who experiences the same issue.
Problem: Avast! free 2014.9.0.2008 was causing Win7 64-bit to hang shortly after login. I wasn’t able to update the program, since the OS would freeze and force a reboot before the update could complete. I haven’t installed any Windows updates recently or made any other software changes, apart from background updates to Avast!
Solution:
[] Boot into Safe Mode and remove Avast! from the startup list.
[] Boot normally, and check that the freezing is fixed.
[] Launch Avast! UI and update the program to 2014.9.0.2013. Restart.
[] Avast! should automatically reinstate itself in the startup list, but it’s a good idea to check that it has done so.
Hopefully these steps will work for anyone else experiencing the same problem.