Have you tried these drivers?http://www.motorola.com/softmodem/driver.htm
I know my old motorola sm56 could be a pain to install,it would take 3-4 tries before it installed and worked.It has to be on com 3 so if anything is using com 3 uninstall it and then install the drivers.If I remember correctly the modem isn't auto detected at all just install the drivers then it should be installed.Thats all I can think of right now.