then a few things
most ( read almost all ) OS's do not let root start the gui
-- bt is a bit different -- it ASSUMES you ARE a hacker --
so you DID fallow
have you read the backtrack webpage ?
or the wiki
but i would NOT recommend this distro for a "new to Linux" user
do a LOT of reading and studying on the backtrack website
is this really what you WANT to do
penetration testing and system cracking
also your hardware specs would help
for instance if this is a OLD computer from 2000 then a gui might never work if you have only 256meg of ram .
--------- most distros ------------
fedora,ubuntu,suse,... the root gui HAS BEEN DISABLED .
so there will be a error if you try to login to the gui as root .
also most os's do not use "sartx" anymore
GDM for Gnome
KDM for KDE4
XDM for others
what desktop did you install ? gnmoe ,kde,xfce,... ?