do you have a boot disk of any kind? during installation they may have given you the chance to make a boot floppy, or you can find various live CD distros available for download online. i would suggest finding something like that which will allow you to boot into a working linux, and edit files on your hard drive. if you use slack live, available from http://freshmeat.net/projects/slacklive/?topic_id=1012
you should just be able to burn that iso on a cd (i'm assuming you are running windows now, or something), pop it in and reboot. use bare.i unless you absolutely have to choose something else. that should work. then you should be able to access your hard drive, but you might have to type
mount -t auto /dev/hda /mnt/hd
which will mount the drive and allow you to access files on it. if linux is on it's own partition, use that instead of hda.
now you can start playing around. well, i guess the first thing i would do is edit your inittab. i like emacs, but you can use kedit or vim or whatever if you prefer, but you want to do this:
this will open up a file for you to edit, and you want to look for a line that says id:4:defaultinit or something like that, and change the 4 to a 3. it sounds to me like your graphics card isn't working, and when boot tries to load the graphical login screen (init level 4) X windows fails. setting the default runlevel to 3 will start the system without X, and allow you to login from the command line. so take the live cd out, reboot, and you should get a login promt. type root and enter your root password then run
/usr/X11R6/bin/xf86config (just type that in and press enter)
and put in all your information, choosing the safe settings when you're unsure. when you're done, type startx.
if that doesn't work, then you probably need to reinstall. hope some of that helps you, and if it doesn't, post up!