Modem trouble

I keep haveing a problem with my modem. If I shutdown my PC and reboot and try to connect to the internet it keeps saying modem not installed right,however; if i remove my modem and shut down and then bring the system back up it reinstalls the modemand it works again, that is till i shut down the PC. Neddless to say every time i want to shutdown my PC i have to uninstall my modem then when i bring up my system again i have to install my modem all over again to get it to work. Please if anyone knows a fix please let me know i have tried about everything.
Thanks.