I Had same problem! Here's solution

he COM port database for XP/2k is at - HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\COM Name Arbiter

The COM port database consists of a 32 integer array, with each bit (0 or 1) representing
whether a COM port is used.
To make the Bluetooth COM port start at a particular number, user needs to block all COM ports
below that number, install BTW to add the Bluetooth COM ports and reset the COM ports which we blocked.
So for example if you open up this Registry key and look at the "ComDB" element you may see
something like the following:

FF 1F 00 00 ... (NOTE: values are in hex, so change to binary where FF = 11111111, etc)

If you convert the values you see into binary (use the scientific calculator), each 0 or 1
will correspond to a COM port, if its free or used. I have COM ports 1 thru 13 that are used.
My registry looks like

FF 1F 00 00 ...

FF -> 1 1 1 1 1 1 1 1
COM PORT 8-7-6-5 4-3-2-1

1F -> 0 0 0 1 1 1 1 1
COM PORT 16-15-14-13-12-11-10-9


Start from top right and go right to left, top to bottom and see which port is blocked or available.
The key is as follows:

1 is blocked
0 is available.

So to block COM ports until COM 5 and start at COM 6, the registry would be
1F 00 00 00.....

1F -> 0 0 0 1 1 1 1 1
COM PORT 8-7-6-5 4-3-2-1

NOTE: You will need to un-install BTW before doing the registry modification. You will block off port 1 - 5,
install BTW, which should then start at COM 6, then free up COM Ports 4 and 5 so you can use them.