Greetingz!
For starters, I'd make sure you're able to get to a console session (Ctrl+Alt+F1, you should see a login prompt).
Second; I'd suggest booting from the install CD and doing the "Media Check" (just to make sure the ISO you burned is completly readable).
Third; Login to a console session as root, then do the following;
init 3 (this should take down the GUI)
cd /etc/X11
cp -p xorg.conf xorg.conf.bakup (makes a quick backup of your X Windows configuration)
Xorg --configure (tries to auto-detect good X Windows settings for you)
Or you can take a gander at the
CentOS5 X Windows Configuration Guide