On a Vista 64 computer 4 G RAM, intel dual core, broadband connection, I use AVG 8 Internet Security (paid version).
Recently, it has come up with the warning "Update Manager disabled"
I can find no way to re-enable it.
Anyone got an idea?

AVG's advice was to find the folder ProgramData\AVG\cfg and erase two files (update.cfg and sched.cfg).
It didn't work!

I had similar happen a couple of weeks ago. To try to sort it all out I uninstalled AVG and then reinstalled- and the reinstalled version wouldn't update automatically either but is now doing so again, so I think that there's something at AVG's end that's gone rather wrong.

Have you tried going on AVG's forum and asking there if anyone else has reported the same issue?

Following their advice a second time seems to have solved it. I'll give it a day or two before I cheer!