Want Windows back...
If you have a valid license for Windows and know the Windows activation code or product ID, you can at any time legally use any Windows installation medium to re-install the Windows for that license. The installation should be recognized as valid by Microsoft upon activation, otherwise you can call in the activation code. A safe copy of XP might be hard to download, but XP is common enough to be found around as a DVD.
Here are Microsoft authorized installation DVDs for Windows 7. Note the "bootable" versions that make it easy.
Using the original installation medium allows you to format the HDD with the number and size of partitions you want and makes it very easy to install linux as a dual-boot.
The Dell system number makes it easy to re-install all the drivers, and you can install or un-install Windows as you please. Windows, some people here don't like it, but sometimes, some need it.