This is the reason why i always use dual boot. If youe machine didn't come with a reinstallation CD, and you have not created a recovery disk before installing W8, than you'll need to do a fresh Windows 7 reinstall.
If you installed over 7 "that's bad news". :sad: I urge anyone attempting this to become familiar with Disk partitioning and Imaging software first. So that you can make the appropriate image backup prior to installing WDP over Windows 7 in upgrade installations. As this would make uninstalling the WDP a feasible and easier process while saving a 0 day backup of your Seven installation files, photos, programs, etc., so unfortunate posts like this can become a thing of the past.