Hi and welcome to the forum
If the PCI modem is causing this sort of issue, I think that the most effective solution is a fresh install of operative system after removing the PCI card.
If this is not a solution for you, then why don't leave the modem installed? If the telephone cable is unplugged you should have no problems
To exclude the USP as cause of the problem, can you try to remove it temporarily for a quick test? If the computer starts regularly after removing the USP, then the problem is not in your computer.