What type of 'pop-ups' do pop-up blockers block?

I have a password prompt that works through window.showModalDialog() and cookies. The script will return the user to a home page if the window is null, ie:

Code:
...
var w = window.showModalDialog( 'pass.html', etc...);
if( w ) {
  //do something
}
else {
  location.replace('index.html');
}
...
Anyway, I have a user that can't get into the 'secured' (heh, keeps the corporate types out) area because they don't get the pop-up and are immediately returned to the home page.
The user has IE5.5, but they're not at my location, so I can't check their PC.
I did confirm that the script works fine in IE5.5 on other PC though.

Given all that, does it seem that there could be a pop-up blocker blocking the dialog, or could it be something else, like browser settings?

Thanks for any help