What John is referring to is changing the boot order in the BIOS.
If you are booting from a flash drive you need to make the first device in the boot order a USB device, the second device should be your hdd.
If you are booting from a disc, you need to make the optical drive the first device in the boot order and the hdd the second device.
If you have this on a disc do the following.
Repair Installation Instructions
Attention: In order to boot from the installation disc you may need to change the boot order in the BIOS so that the optical driver is the first device in the boot order, and the HDD/SSD is the second devcice.
1. Place the installation disc in the tray of the CD/DVD drive, close the tray and restart the computer.
2. You will be prompted to press any key to start the installation, do so.
At this point the setup process will load files, this will take several minutes.
3. You will now need to choose the Language, Time and currency format, and Keyboard or input method that you'd like to use.
After this is done click on Next.
4. Click on the Repair your computer link at the bottom-left of the Install Windows window.
This link will begin the Windows 7 System Recovery Options.
5. System Recovery Options will now search your hard drive(s) for any Windows 7 installations. This will take several minutes.
No participation is required on your part at this time, wait till it has finished and the next window opens.
6. Choose the Windows 7 installation that you'd like to perform the Startup Repair on, then click on Next
7. Click on the Startup Repair link from list of recovery tools in System Recovery Options.
For a future reference, there are several other diagnostic and recovery tools available in the Windows 7 System Recovery Options including System Restore, System Image Recovery, Windows Memory Diagnostic, and Command Prompt.
8. The Startup Repair tool will now search for problems in the system files.
If Startup Repair finds a problem with any system files the tool may suggest a solution which you will need to confirm, or may solve the problem automatically.
9. Startup Repair will now attempt to repair whatever problems it found with system files.
Note: If Startup Repair did not find any problems with system files you won't see this step.
Important: Your computer may or may not restart several times during this repair process. This is normal, you should allow it to continue until you see the Restart your computer to complete the repairs window.
10. Click on Finish, this will restart your computer.
It is possible that the Startup Repair will not be able to fix the problem. If the Startup Repair tool determines this, it may automatically run the the repair after your computer restarts. If it does not automatically run the repair but you are still having problems with Windows 7 repeat these steps to run Startup Repair again manually.