Try this :
1. Run a memory diagnostic (eg memtest86 - http://www.memtest86.com/
) to eliminate any RAM problems.
2. Go to Run... on start menu & type msconfig to start System config util. Click on 'startup' tab & disable all the entries. (you can probably leave scanreg, systemTray & task monitor enabled if you like)
3. Reboot & surf the Internet for a while (I realise that you will have no AV protection etc cos Nortons will be disabled at startup, so don't download anything). If you don't get any errors, re-enable the startup items one at a tiem to eliminate which one is causing the problem.
4. If you still get errors, you could try uninstalling the modem driver, remove the modem physically & connect it to a different PCI slot, let Windows redetect it & install drivers.
What com port is your modem assigned to & what IRQ is it using?