My response is (informed) speculation based on years running an ISP in the days of modems, and also having used [ and sold ] X100P cards - however I have not run across your problem - I answer because I suspect most people havn't come across this - particularly as with the rise of VOIP, X100P cards are - at least in my part of the world - largely a thing of the past.
An X100P card is actually just a specific Windows / Soft Modem, ie a modem with cut down firmware - which Digium wrote a driver for and can be controlled sufficiently by Asterisk.
The problem is probably that you don't have any free IRQ's to be assigned to the modem. The first thing I would try is loading your BIOS and disabling devices which you don't use - for example Floppy disk drive, but more importantly, any serial or parellel ports if you have them.
If that fails, you should try adding the command "pci=routeirq", or, if that fails, "pollirq" to the boot line of your kernel. (If you don't know how to do it, thats a seperate question - we would need to know the distro and version you are running - and, if you know it, which boot loader). Here is an old, but possibly useful post related to this paragraph.
