problem connecting huawei e1731 unlocked modem

My huawei dongle is being treated like cd rom and mass storage,it is not switching to modem mode even after installing all drivers.
Facing the problem in windows 7 ultimate 32/64bit.
My modem model-Huawei mobile broad band e1731(e173cs-1)
laptop-dell inspiron 1525.
Please do help.