Is your PC a retail brand...ie HP, Asus, Dell and so on. If so, you should have a partition called the D: drive that has the OS hidden to set back to factory condition, the day you first turned it on and set it up. There also should be some sort of branded recovery software in your programs list, under the brand of computer.
This will only work if you upgrade from windows 7 to windows 8. If you do a clean install, which is the preferred method....less problematic, you will loose everything to include the hidden partition to restore windows 7.
There is a work around if you do go the clean install route of windows 8. You will need to get windows 7 ISO, free download, bit specific and OS specific and reinstall windows 7 or order the recovery OS from your PC's website. I've ordered several from HP and if I remember correctly they were $20 each. Keep in mind, either of these options wont have the bloated/add on software that came with your system. The PC's website might have all that software to download if you so desire.