What you are experiencing is a known issue in WebCT Vista. Here is a way to get around this issue:

1. For Internet Explorer Web barticleser, on the Tools menu,
select "Internet Options."
2. On the Advanced tab, under the Barticlesing section, click to clear the
"Show friendly HTTP error messages" check box, and then click OK.

Also, be sure that you are not running any barticleser add-ons
such as the Google toolbar or Yahoo! companion and that you do not
have a pop-up blocker running.

This should allow you to log into your WebCT Vista course.