If you have a valid product key for Windows 7 on a sticker somewhere on your computer, or on a installation DVD that came with your computer, then you can re-install and activate Windows 7 with no problems. Here is link to a great tutorial on the subject.
System Manufacturer/Model Number Customized build from CyberPower OS Windows 7 Home Premium 64bit SP 1 CPU Intel i5 2500k Motherboard Asus P8P67 Deluxe Memory 8 gigabytes Corsair PC3-12800 DDR3 Graphics Card EVGA GeForce GTX 460 superclocked Sound Card Integrated Monitor(s) Displays ViewSonic 23" LCD Screen Resolution 1980 x 1080
Keyboard Logitech USB Mouse Microsoft 3 button PSU Coolermaster 1000 watt modular Case Coolermaster HAF X full tower Cooling Coolermaster Hyper 212 plus Hard Drives 120 Gb Samsung 840 Pro SSD
120 Gb Kingston Hyper X SSD
1 Tb WD Caviar Black HDD Internet Speed download 1.5 Mb/sec upload 300Kb/sec
Since you didn't mention the brand PC you'll need to check for which key to tap at boot to run Factory Recovery which sets everything back to factory condition so that the new owner can set it up with their User name and information when they start it up.
If Factory Recovery won't run or you didn't make the back-up Recovery disks then get the superior Clean Reinstall Little Jay provided above. You can delete all partitions during install if Recovery won't run or Dell or HP Diagnostics (Tools) partition won't run from boot.
An image will still have your data and Programs on it, so running Factory Recovery might be the better solution since it puts it back to the condition it was when it came from factory - like new for the new owner. It's the same as an image but doesn't have your stuff in it.
Did you wipe the Recovery partition? If not have you tried running it from boot?