I just acquired a 3com 4200G switch, and am having problems logging into it. I'm using a console cable, and can see the switch boot up and prompt for usename and password. The default password (admin and "nothing") does not work, nor does admin admin.
A site here: http://www.urtech.ca/2011/12/solved-how-to-reset-the-password-on-a-3com-swictch-4200g-3848/
has instructions on resetting to default, but that does not succeed either.
Here are some other details of the switch that may be relevant:
4200G 12 Port BootROM, version 1.00
creation July 3 2006