If you installed windows after linux then windows will have trashed grub. What are you using as a boot mgr? Can you boot successfully into Linux?
If you are able to boot into Linux CLI and just can't get X to work then xorg is not configured correctly.
You say that you used Yum so you must be using Fedora.
Save the xorg.conf that you now have to a safe place then run as root
Answer the questions with the correct values. Run it several times perhaps lowering the resolution and depth until you get X running.
When you get X going save that copy of xorg.conf and then tweek xorg it until you get what you want. You can always overwrite it with the saved copy if you mess it up. If you saved a good xorg in your /home/<user> then as root
Code:
cp /home/<user>/xorg.conf /Xll/xorg.conf
substitute your own parameters