Computer lock up

Sounds curiously to me like one of two things:

Errant Wake On Ring behaviour, possibly because the modem is sending a wakeup/boot signal to an XP that doesn't expect it.

Or

An IRQ drama possibly, with the modem sharing an IRQ with another device. (Dashboard theory) Perhaps disabling boot on IRQ 3 (or similarly named option) in the BIOS may help.
Additionally, I've seen some inexplicable stuff from internal modems, so if that's the case it may help to shuffle it's PCI loction.

AND, this just in from the depths of my memory:
When a modem detects a ring, it sends the string "RING" to the com port (supposing the com port is in communication state with the modem at the time.)

It's hard to answer without playing around withthe PC in question - so all I can offer is ideas.

When you installed XP did you do an upgrade or clean install? If it was an upgrade did you keep the FAT32?

If this is the case I'd recomend that you wipe the drive in question and do a clean install of XP this way you will not have any stray drivers hanging around to gum up the works. I've run across numerious problems with the upgrade version of XP particuarly if you have kept the FAT32 rather than switching to the NTFS file system. While XP will run on a FAT32 file system it isn't all that relaible and can cause some strange problems that seem unrelated to anything in particular but a clean install almost always fixes the problem. I am of course asuming that all of your existing hardware is XP compatable. You can check this by visiting the MS web site and downloading the test tool for your hardware. This is not a definitive test as some devices will pass the test but will still not fuction correctly if at all when run under XP but it is a start.