I suggest you reboot your computer into the Windows Recovery Environment command prompt. Information on how to access it can be found here:http://www.bleepingcomputer.com/tutorials/windows-7-recovery-environment-command-prompt/
The best method is to tap F8 when your computer starts and when you get to the boot menu, select Repair Your Computer
. If that is not an option, then you need to restart your computer with the Windows 7 DVD inserted and boot off of that.
Once you are in the recovery command prompt, type:chkdsk /F c:
and then press enter.
Chkdsk will now scan your drives while they are offline. Let's see if it finds anything that could be causing Windows 8 to not install.
As always, make sure you have a backup of your important data before running any program that could alter your hard drives. For the most part, you should be safe with chkdsk, but better to be safe than sorry.