If your trying Windows 7 or Vista 64bit, you'll find Belkin don't have 64bit drivers. However, Belkin don't make the drivers anyway. You can checkout this blog post on how to get it working under 64bit Windows: http://www.clearchain.com/blog/posts/how-to-use-a-belkin-f5u409f5u409-cu-usb-to-pda-serial-converter-in-visawindows-7-64bit-mac-osx-10
Belkin F5U409 USB to Serial adapter driver for Windows 7
Belkin F5U109 USB to serial adapter
Belkin F5U109 (serial to usb adapter) driver
If your trying Windows 7 or Vista 64bit, you'll find Belkin don't have 64bit drivers. However, Belkin don't make the drivers anyway. You can checkout this blog post on how to get it working under 64bit Windows: http://www.clearchain.com/blog/posts/how-to-use-a-belkin-f5u409f5u409-cu-usb-to-pda-serial-converter-in-visawindows-7-64bit-mac-osx-10
Found it at: http://clearchain.com/downloads/Belkin\_FU409/U232\_10.2.zip