What is probably happening

What to do

IRQ conflict, most likely between the modem or sound card and the mouse driver

Look up the IRQ settings and note what they are. If you don't know how to do this, see Windows Help.

Then, change the setting on your modem through the range. If the problem does not go away, set your modem back the way it was and repeat with your sound card or other PCI device. Your ultimate solution may be to replace PCI cards with USB-driven devices. A USB mouse is a good place to start.