Well, use factory restore. It will erase it to the brand new condition.
If your operating system is VISTA / Win 7, tap F8 key when you see the Dell logo. It will take you to advanced boot menu options which include `Repair your Computer'. Select that option and enter your password if asked, then you will see a list of options like `system restore', `command prompt', etc. The last option says Dell PC restore or something similar to that. Select that option and follow the screen.
If in both cases, the Factory default doesn't work, then your laptop's factory image defaults partition have vanished (i.e., Corrupted / Deleted). So you have to reinstall the operating system with the Operating System CD / DVD shipped along with the laptop. Follow the link below to install windows XP Pro on your own.
Start the computer.
As the computer starts, press the F8 key until the Advanced Boot Options menu appears on the screen.
Press the (Down Arrow) to select Repair Your Computer on the Advanced Boot Options menu, and then press Enter.
Specify the language settings that you want, and then click Next.
Log in as a user who has administrative credentials, and then click OK.
Click Dell Factory Image Restore.
In the Dell Factory Image Restore window, click Next.
Click to select the check box for Yes, reformat hard drive and restore system software to factory condition.
Click Next. The computer is restored to the default factory configuration.
When the restore operation is completed, click Finish to restart the computer.