AVG 2011 Free edition causes problems for Windows 7 64-bit users

A virus definition update for AVG 2011 Free edition seems to be causing problems for Windows 7 64-bit users.

[UPDATE: I'm hearing from some users of the non-free version of AVG 2011 who say that they are also affected.]

STOP: c0000135 The program can't start because %hs is missing from your computer. Try reinstalling the program to fix this problem.

If you're affected, three possible solutions are presented here. Also listed is a fix for those who have installed the update but not yet rebooted their systems.