I'd had the same problem too and I tried couple days to solve it. It works for me, maybe it's worthy for you to give it a try.
Setp1: Open the "Device Manger" then click the "Modem" item.
Step2: Click the "Com Port Numer", you'll find out that many com ports have marked "IN USE". Just don't fucking care about it, choose any com port you like. Take "COM 9" for example, choose com 9 and click "OK".
Step3: After that, a POP-UP warning window will show up. Just click the "OK" anyway.
Step4: After Step3, you need to change the modem com port to other com port you want. Now, the COM 9 is available for ActiveSync now.
Step5: If you need more com ports which are unavailable, just repeat the procedure above again.