I develop s/w for PPCs and get this sort of thing all the time. My usual checklist is:
1) Turn handheld (HH) on and off.
2) Remove HH from cradle and stick back in again - this is a really good tip to solve intermittent connectivity problems.
3) turn PC on and off
4) Uninstall and then Reinstall ActiveSync, making sure to be logged in as the same user as when you try to use ActiveSync
5)
Hard reset the HH - see the manual, if you don't have the manual look on
Search HP United Kingdom - Results - 'ipaq hx2100'. Note: Hard reset returns device to factory settings - you will lose any data.
6) Try to isloate which bit isn't working - try somebody else's cable, try somebody else's iPaq, try somebody else's PC. Do these one at a time.
You shouldn't need to change any settings on the HH for AS to connect. On the PC you have the dialog with the "Get Connected" button on it. You should only have to make sure that the "Allow USB" box is ticked (obviously).
My AS version of preference is 3.7.1 at the moment. That should work ok with the 2100.
ActiveSync is getting better, but is still a PoS.