Firstly, its commendable you're switching back to the licensed version.
When booting, if you press F12 (for boot options on Dells), you should get a menu that gives various boot options, the last one of which will be something like 'Factory Restore' or 'Recovery Partition'. Choose that, and it should initiate the restore.
If that doesn't work, and if the underside of your laptop has the OEM license sticker, you can clean install Windows 7 Home Basic from the appropriate disks (if you don't have a disk, you can ask Dell for it, or even borrow a disk from a friend).
If you do a clean install, you might want to visit the Dell Support site for your laptop model to download the latest drivers and applications (for things like Wi Fi control, etc.).