Change ISPs? NetZero uses its own dialer, not the one built into Windows.
Look into whatever app NetZero installs to idiot-proof your connection to
them and see if you can turn off this "feature". I doubt that you can. I'm
betting they have some incompatibility between their dialer and any Sleep or
other APM feature (like it won't resume when you come back from your
bathroom break) and their solution is to turn it all off when the dialer is
active. Does it return the settings to what they were when you're finished
and the dialer has unloaded?
Of course, I'm totally guessing, here. I'll bet you'd get another answer
from NetZero, though which would be more correct.....???? Their "Help" makes
no mention of the issue.
Oh, and BTW... Have you updated your NetZero software to the latest? Version
8.5?
(No, I know of no way to make Vista stop it from happening.)