John:
Thanks for your reply. Unfortunately, Paint keeps trying to scan from a printer which is not currently connected (HP5510) and I can't convince it to find the 6500.
As for your other ideas, I've tried them all, including using different routers, rebooting the router, uninstalling and reinstalling the drivers and software, downloading the most recent version of the software and drivers, etc. Generally speaking, except in the most unusual of circumstances, I only have one printer turned on at a time.
To me, it appears to be an unresolved a software/firmware problem.