What version of tomtom are you using Doddel ? If you are using the old version 3, the tomtom programmes is reluctant to share the gps with another programme.
If it's the new Ipaq 5935 all-in-one, we had trouble with Les Henson's one and couldn't get MM to read the satellites and then lost the tomtom's ability to find satellites too . It needed 3 sets of port settings and no combination of them could be made to work. Internet searches turned up a couple of sets of port settings but neither worked for Les' 5935.
Do let us know how you get on
Mo