I dunno if anyone here can help, but if you can’t there’s no hope at all! 
My sister just sent me the following email message:
I don’t know what to tell her except to see if there’a a help line to call in case of emergency. Could this be a problem with their computer’s settings or is their ISP just busy? (I doubt it’s a virus – those things are way overhyped.)
I’ll let my sister know what to do as soon as someone gives me some good advice.
Thanks in advance.
~~Baloo
Sounds like its her computer. There are a number of things which could be causing this. NetZero has a support section on their website, with an entry for this error:
http://netzero.net/support/errors/EW04_error645.html
Other ISPs have these suggested fixes (listed in above link) on their sites as well, including wiping all .pwd files from your hard drive. This is because the pwd files (which store password information) sometimes get corrupted.
(taken from http://www.iinet.net.au/help/faq/winerrors/error645.html)
To remove the Windows .pwl files:
[ul]
[li]Go to Start then Find and click Files or Folders In the Named field type *.pwl[/li]
[li]In the Look in field, select C: or the drive where Windows is installed.[/li]
[li]Click Find Now[/li]
[li]When Windows has found the .pwl files, click Select All from the Edit menu, then press Delete (or DEL) on your keyboard. These are generally harmless files, but it is recommended that you DO NOT empty your Recycle Bin until all the software is fully functioning.[/li]
[li]With all .pwl files removed, perform a complete shutdown of your system and turn the power off. Wait about 10 seconds and restart the computer. Depending on how you setup your system, you may be prompted for a username and password at start up. If this happens, enter a username in all lower case characters (8 maximum) in the Username field and leave the Password field blank. Once you click OK, you will be prompted to confirm the password. Leave this field blank and click OK again. You should not be prompted for that username and password again.[/li]
[li]Note: If you add a password at this time, you will be prompted to type a password every time you start your computer.[/li]
[li]Try connecting again.[/li][/ul]
The pwd file(s) should be recreated when you enter your username and password while connecting.
Hope this helps. 
From time to time, every ISP has down time. Wait another day & see if things get better. This always works for me. IF you modify your settings instead, you very well may not get back on line when your ISP is back on its feet.
Also, restarting the computer would be the first thing to try.