can't read com ports compaq armada

I have a compaq armada 7730 mt that I use to download a radon monitor and a temperature logger. I recently bought another one for a back-up, since this one has worked so well, but is getting a little bit flakey sometimes. Trouble is, the computer can't read the devices, as if the com ports don't work. I recently formatted both hard drives and reloaded win98 from the same disk on both computers. The older computer will read the devices, the other one will not. I have run the diagnostic program that is in the computer, and it says everything is fine. Any suggestions? Thank you. Bill