I'm not clear, you say you can't 'open' IE7 or FF, is that exactly what you mean or do you mean you can't browse and sites when using them.
If you literally mean you can't physically open them, then there is something going on in the background, that appears to be conflict. Typically two AVs trying to hook a file to scan it, so what is strange is that this only effects IE7 and FF and not other executable files when you try to run them. So there is certainly something strange going on in your system, as it has all the hallmarks of a conflict yet it is only when opening browsers
To get round the problem, you could pause the standard shield open the browser, using about blank as your home page so it doesn't connect immediately and once open resume the standard shield so you are protected.
This isn't a permanent solution but a test, can you then browse or do you experience other issues and if so what ?
What were the errors when you tried to manually delete what avg registry entries you found ?
I suspect some sort of permission error, in which case you would have to take control of the key, e.g. Right click on the key, select Permissions, and ensure your user account is allowed Full control, that should allow you to delete that avg entry in the key.
