I got a switch 3CR17500-91, but no password :-(. Luckily the knowledge base of 3com was able to help me (3KB10785)
* Login at CLI -default serial 18600, 8N1- using username/password recover/recover
* Hard reset -eg poweroff-poweron-
* Wait till you see
Enter the new password for the Administrator:and enter your new password
* Now able to login username ``admin'' and your newly typed password :-)
* You are back in business!