I have also had issues with getting this to work on vista and tried adding the handler to iis without success. However I have found that making sure that the webconfig file contains
<add verb="*" path="*.wgx" type="Gizmox.WebGUI.Server.Router,Gizmox.WebGUI.Server,
-->Version=3.0.5701.0,Culture=neutral,PublicKeyToken=3de6eb684226c24d" />
if this line contains something different which it will if you use the enable webgui function your project will give an error in IE explorer.