At the command line, log in as "root", then type:
sed -i "s/id:3:initdefault:/id:4:initdefault:/" /etc/inittab
then reboot, you'll then be able to choose your preferred desktop or windows manager in a menu.
you do that, please create a regular (non privileged) user typing:
When you are done type the aforementioned command, reboot and choose to log in as a regular user (the one you just created).
To know more, visit http://docs.slackware.com
I suggest that you post your questions about Slackware in the Slackware forum http://www.linuxquestions.org/questions/slackware-14/
to get more relevant answers.
PS I just corrected the first command, sorry if you already typed it.
PPS Corrected again, thanks Brian.