Thanks for your help.
Windows did not detected it as a new hardware instead there is already "Standard Modem" installed.
So, I disabled auto detection and manually installed 3 Com US Robotics Modem. It is working fine. But, now there two modem installed on the same port COM1. (both standard & 3 Com US Robotics).
I deleted the Standard Modem entry and it is again there after the restart.
There is also an unknown PCI device in device manager which can not be installed.
I am using Dell Optiplex GX150 Computer with following devices:
External 3 Com US Robotics 56K FaxModem
HP Laser Jet 1150 Printer (Installed on LPT1 instead of USB Port)
External CD Writer (CREATIVE SB1394)
Just trying for solution, I have found that there is following port sharing conflict:
Standard Universal PCI to USB Host Controller PCI\VEN_8086&DEV_2442&SUBSYS_00000000&REV_02\3&29E 81982&0&FA
Please guide me further.