You might go into a little more detail about how you are installing. Are you doing a download install over Windows 7 or do you have the Windows 8 DVD? The more info about the process and your system, the more likely someone might see a potential problem.
You might run a System File check on Windows 7 to make sure it is in good shape prior to running the upgrade. Open an Administrative command prompt and type sfc /scannow and let us know if it show anything that cannot be repaired.
As Saltgrass asks - Are you trying to install from Windows 7, or from an install DVD?
If the former, Have a look in the Windows 7 services (run Services.msc)
Scroll down and change the Windows installer to Automatic.
See if that works for you.