The most stable channel is 6. With 40MHz this will work almost flawlessly together with other networks even when they are on the same channel. hence draft-n works with multiple channels to get a higher speed.
20MHz networks are b/g networks so again, channel 6 and 40MHz are the best settings
What network card are you using? are you sure it's enabled in the windows control panel, and do the settings allow connecting to b/g networks?
different firmwares shouldn't matter btw, since the drivers are precompiled binaries so there can't be any editing. Only DD-WRT has custom wireless drivers thanks to some sort of contract with broadcom.