Have you checked to make sure the drivers are up-to-date? If they are and this is still happening, at a possible virus/spyware problem or something else. After checking drivers, do a House Call virus scan at
www.pc-cillin.com, if this comes up empty, try some of the malware steps posted here:
Basic Malware Removal
If
that doesn't work, let us know. Otherwise, a reformat might be in order...
P.S. How long has this been happening, and has it been happening since you've been using the modem and dial-up service?