If you have Pop-Up Stopper, a stand-alone pop-up blocker software, an icon will appear in the bottom right hand corner of your screen.
If you hover your mouse over it, it should show the name of the software.
To disable Pop-Up Stopper:
Right-click on the Pop-Up Stopper software icon.
Un-check the checkbox next to "Enable Pop-Up Blocking".
Re-enable the software when you are done using Vista WebCT.