See the Dell support page Restoring Your Computer's Software to the Factory Settings
It lists the keys to press when booting to restore to the factory configuration.
To save time going to that page and clicking a couple buttons, here is a copy of what it says:
A. Backup all data stored on the system.
B. Except for the monitor, keyboard, and mouse, remove all external peripherals.
C. Using PC Restore in Windows XP or Media Center
1. Turn on the computer.
2. When the Dell splash screen appears during the computer startup process, press and hold <Ctrl> and then press <F11>. Then, release both keys at the same time.
3. In the Dell PC Restore by Symantec window, click Restore. Alternatively, press <Tab> to highlight Restore, and then press <Enter>.
4. When a warning message appears stating that all data will be lost, click Confirm, or press <Tab> to highlight Confirm and then press <Enter>.
The Progress window will appear. Once the restore process starts, Dell PC Restore by Symantec usually takes about 8 to 10 minutes to finish.
When the restore process is complete, the following message will appear:
The system recovery process was successful.
5. Click Finish or press <Tab> to highlight Finish and then press <Enter> to restart the computer.
The software is now installed as it was when the computer was first received.