Did you reboot the PC? Sometimes the FBOOT???.EXE application manages to completely lock the USB subsystem on the PC. On my system the lockup happens immediately and invariably (ie: I can't use the bootloader at all) - but that's not a problem since the ISP hardware programmer is much easier to work with.
Apparently the FBOOT7.EXE application is a MsDOS application (not to be confused with a Console application, it's MsDOS application that would happily work with MsDOS 6.0 for example) and that's why it only works with the COM1-COM4 ports and that's probably how it manages to "fly under the Windows radar"
Maybe someday someone writes a Windows version of that application so it works properly on the Windows system (that way it would also work with COM port numbers grater then 4).