On the "average" PC there are at least three Programs that will dial up atomatically unless configured not to do so. They are:
Windows Automatic Update (waulaunch.exe I think)
Zone Alarm (Pro) will try to find a newer version periodically
Anti virus software (eg aulaunch.exe in McAfee)
Add to these, RealPlayer, Acrobat reader, etc etc.
So I suggest you go through all the likely suspects on the offending PC(s) and see if you can de-activate any automatic update schedules.
As both PCs are XP, it is a simple matter to look at Task Manager and see what processes are running. They will give the game away.