What system is this on? You are getting the message most likely because you installed a Plug and Play device and your system does not fully support Plug and Play. If your modem has jumpers, you might try jumpering explicitly for the resources it should use, and this might clear up the error.
I replaced my Sportster 28.8 with a USR 56K Voice Faxmodem, and now in the boot sequence I see "NVRAM invalid, NVRAM cleared." What does this mean, and how can I get the issue cleared up, so it doesn't come upduring the boot sequence?