If you have root access, go look at line 1482 of /etc/httpd/conf/httpd.conf and see if it is:
ErrorDocument 401 /401.shtml
if its not, then see if this line is in the file at all...
If you dont have shell, then you might want to check the file size on the 401, as IE requires a certain size for error pages to be displayed and not use its own. (guhg! i hate ie doing that).
Try adding 500-1000 spaces at the end of the file, to fill it out