I wrote an ActiveX Control in VB6 for Windows using WindowsNT 4.0 Workstation as the development platform and we don't have any problems with serial ports using it on Windows XP. It even works great in VISTA. Albeit, we had to make sure that we didn't put any of our controls in the system folders: just put 'em in our Program Files folder.
I would say that the problem is not in the serial ports, it's in the software.
If you really want to use the CM11A on XP, I'm sure that there are several here that would help you to tinker around and get it working on XP.
Hank