Check your IRQ sharing.  The older MoBos don't handle resources very well, you may have a conflict with your modem, NIC, and USB card.

If they are not trying to use the same IRQ I think the next thing I would do is move the NIC to another slot and see if it goes away.

You might also check the BIOS and see if it is assigning an IRQ for a PCI video card whether one is installed or not.  Some had that setting available and you can free up an IRQ (since you have an AGP card) if that is the case.