Com Port Problems

Hi
I’m having trouble with a new PC when I connect to the internet, the modem appears busy.
The only way to solve the problem is to unistall the modem, restart and reinstall it.
I’ve disabled the XP Firewall, but have Norton Firewall. I used this Firewall with no problems on my old PC.
The modem is an external one and I’m on dial up.
I’ve asked this same question on three other forums and no one can help!