If you can prove you purchased the switch with the web images on. then Cisco should be able to send you a copy. Unfortunatly Cisco licence you to use different versions of OS for the switchs, some do have and some dont have the web interface.
ITs a bit like purchsaing Windows XP home, and then wanting to download the Pro verson for free...
There are places to get copies of what you need, but as it is Ileagel I can't help you find them.
Sorry but that is the rules of the board, as wel as the law..
Ps. if you have another 3550 with the web interface files on it you can copy the files across!!! ;)
