If you're registered with Dell as the owner, and you live in the US, you can request replacement disks here - elsewhere, call your local Dell Tech Support line:
Otherwise, you're pretty much on your own. There should be a COA sticker on the laptop telling you what version of Windows the laptop is licensed for (XP Home, XP Pro, etc.) - what you want is a Dell Operating System Reinstallation CD with the same version (the disks are not model-specific). I've seen them for sale on Amazon. Worst case, though, you need to buy a copy of Windows.