Do this, at the command prompt, type this (as root) -
xorgsetup
It should detect your display card and mouse, and generally everything should go well. If there is still some trouble, try
xorgconfig
This tool will proceed to ask you some questions about your mouse, monitor, graphics card etc.
If yet there are troubles, do post your /etc/X11/xorg.conf file here, and do llok at the docs here -
http://www.slackware.com/config/x.php