"The share name does not exist on the server"

Recently, when I boot up my MacBookPro, a dialog box appears asking me to enter my name a password for the server "192 etc (whcih is my iMac". However, I've never set up a password. Even when I try to connect as a guest I get denied. I have no problem connecting with my Macbook Air. I've checked all the sharing options and they seem fine. Any thoughts?

Thanks

Philip