Blecher,
Yes, if it does not have a physical port and is turned on in BIOS that could cause it. It is more likely that it is a bad or misconfigured modem, which is also a serial port device. Depending on the type of modem it may never pass i.e. a Win Modem does not use IRQ and Base I/O addresses like a real modem or serial port does. It can not be tested under DOS only under windows after the driver has been loaded.
-Fred