Follow the steps below and it should allow you to restore the computer back its original Windows 7 state. It's your choice as to whether you want to attempt to save files you've created or perform a full, destructive recovery. The destructive recovery will take it back to a factory state but your important files will no longer be there.:

1. Shut down the Computer.
2. Turn on the Computer and start pressing the “F8” Key on the first Dell logo. Keep on pressing the “F8” key on first Dell screen until the Advanced Boot Options menu appears on the screen.
3. Highlight "Repair Your Computer" and hit enter.
4. Specify the language settings that you want, and then click Next.
5. Enter your Password and click ok.
6. On Choose a Recover Tool Window
Select: Dell DataSafe Restore and Emergency Backup
7. You Get:
a. Restore computer and preserve my new or changed files
b Select other System Backup and more options
8. Select: Select other System Backup and more options
You get:
a. Restore my computer
b. Backup Files and folders
Select: Restore my computer..
Confirm the restore and continue..
Select: Factory Image restore
9. Choose Restore without preserving new or changed files and click on Next.
10. Click on Continue.

The link below also describes similar steps for accessing the Recovery Partition on your Dell machine.

Hope this helps.