Fred,
First, that isn’t happening here - Windows 7 Pro, 64 bit. Firefox 25.0.1 Avast 2014.9.0.2008 Free.
If I understand what you have said correctly, as soon as you log into Win 7, Chrome opens at that URL.
Did you perchance miss the “opt out” box when you installed/upgraded Avast that stops it installing Chrome?

It might be an opportunity to find out what the tick box at Avast->Settings->Trouble Shooting->Do not use Chrome as Avast Default Browser does, otherwise I would uninstall Avast using Avastclear.exe in safe mode, if Chrome is installed uninstall that, cleanup, reboot and then reinstall Avast.