jack, if you quit the AVG control centre, AVG is still scanning your system. All this option does is remove the AVG icon in your notification area so as you don't have access to the control centre.
When installing s/w, I disable AVG by opening the control centre, double click on the "Resident Shield" box and untick "Scan Program Files" and "Scan Documents". You'll then get a pop-up saying that "Residential shield is loaded but is not fully functional".(If you're using XP SP2 and the security centre is configured to monitor your anti-virus status, you'll get a pop-up warning from this as well).
Doing this stops AVG scanning executables, which is what you want when installing s/w.
I do this when installing any s/w, and have never had any problems.
Don't forget to turn it back on after you've installed you s/w.