With the Dell laptops, Dell stores an image of the computer OS in a hidden partition, for just such situations. That way you will have the OS that came with the laptop freshly installed.
You didn't mention what model of laptop you have, but here are the steps from an Inspiron 1505.
To use PC Restore:
1 Turn on the computer.
During the boot process, a blue bar with www.dell.com appears at the top of the screen.
2 Immediately upon seeing the blue bar, press <Ctrl><F11>.
If you do not press <Ctrl><F11> in time, let the computer finish starting, and then restart the
This will lead you through the process of a fresh installation of the original Windows OS.
With OEM operating systems like the Win-7 Ultimate from an HP machine, the OS product code is tied to the MB and it will be registered as a Non Genuine copy.