User’s frequently report receiving an error about an “Invalid Update Control CTF File” when attempting to update AVG 8. This error originates when the CTF file AVG downloads as part of its update process becomes corrupt. The fix is to delete the CTF file and thereby force AVG to re-download a non-corrupt version.
There is a forum post on the AVG web site which (briefly) details how to manually delete the CTF file. You can follow those instructions, or download the following file which will automatically delete the necessary files.
Automatic fix (Windows XP/Windows Vista): CTF_Fiz.vbs
If deleting the CTF files does not resolve the issue, a reinstallation of AVG usually will.