Hi,
Couple of things to check please.
1) TOMTOM needs to be installed to your IPAQ not your storage card
2) Install TOMTOM and a small map (both to the internal memory) test it this way to see if it locks up.
3) Apparantly there may be issues with SD cards made in Taiwan so if you have an SD card from there then no. 2 should eliminate that.
4) A ROM update is available for your model at
http://h18007.www1.hp.com/support/fi.../105_5721.html
(It will delete all information on your IPAQ)
5) A Self test Application is available from the above site also.
When you say it locks up, does a soft reset work ?