Problems using Huawei E160 Broadband Modem

Hi
I am having a problem with my Huawei E160 Broadband modem. The Modem is unlocked so that it is not fixed to just one network. Unfortunately anytime I fix any other SIM card on it, it brings following error message,
'YOU CAN'T' CONNECT BEFORE YOU REGISTER AN AVAILABLE NETWORK'
When I try to register the network it asks me whether I want to register the previous networks, if I say Ok, it says no network found.
What else am I surposed to do for it to connect to other networks after the unlocking?
Regards
