Modem likes to reboot computer

[Jeff7]

Curious problem: the modem will occasionally, without any warning, reboot the computer. I know it's the modem - the reboots only happen when I'm on the Internet, primarily when I check mail in Juno. As soon as it finishes downloading the mail, about 1 out of 6 times, the computer reboots - I don't mean standard Windows shut down style, I mean instant reboot, as though the reset switch had been hit. Could it be the modem, or the quality of the phone line? Or something else?
I've got a Phoebemicro CMV56KRA modem based on a Rockwell chip. Any ideas?

[Serge]

Sounds like an exploit.. are you running any anti-virus with lates virus definitions?

[Jeff7]

Yup, got Norton AV, v5.0, and I always keep it up to date. This problem has been happening for quite a while now - several months; it seems to be getting worse.
Could it be because of too many devices being connected to one phone line? Let's just say, this one line is probably at its limit - 6 phones, 4 modems. And another line isn't an option at this time. Do signal amplifiers exist for this, if that is neccessary?