If you're doing this through IIS, go to you Internet Information Services console, right click on the Web site in question and select "Properties". One of the tabs in the resulting box will be a "Custom Errors" tab (or something similar). You can use that to set up custom error pages. This is more robust than trying to alter all of your scripts to output the correct page.

Cheers,
Ovid

New address of my CGI Course.
Silence is Evil (feel free to copy and distribute widely - note copyright text)