I'll give first answer a thumbs up +1.. but it may be simpler than that.
*read through and plan before you actually do anything! *
You didn't say which model XPS. They have several generations of XPS laptops and desktops.
Still - the general process goes something like:
1) backup anything important that isn't already done.
2) gather all your disks together - anything that came with the machine, and try to get any newer drivers or updates, and any software you installed - and want to re-install.
3) Check the dell support website [ support.dell.com - link 1 ] and put in your System Tag. They will take you to a list of all drivers, software, manuals, and updates that have been issued since the machine left the factory.
4) While you are on the Dell website - they should tell you with a nice tutorial how to go about re-installing your OS. This is one way.. but Dell lists at least 2 dozen slightly different ways.. check for your specific model. (link 2)
* Dell might be able to help you out with a OS disk if you cannot find yours. Contact customer care (not tech support right off), and ask them for both OS re-install and Driver re-install disks.
I find that for simple requests like this, Customer care can sometimes be easier than technical support. The techs have to follow a certain script - and in your case, nothing is really broken.
5) There may be a system restore partition on the drive. If you didn't replace the disk, or did not FDISK it .. that partition may still be there. F11 or F12 on boot may give you the option to restore the original operating system / or the "out of the box experience" (oobe).
(... again, this should be in your system documentation that came with it... or in copies of documents available on the support site)
Finally, I have done this on dozens, maybe hundreds of machines.
If you have a restore partition still on the drive, that will save you a lot of time.
If you have to do this all manually - be prepared to spend the better part of a day working on it.
The work is not complicated, but keep a checklist so you don't forget ( or double up) any steps, and know that some installs will take several minutes or even hours. You can usually just walk away, get some coffee ( or dinner, or water, vodka, tea) or even let it go while you sleep. When you come back, look for the next prompt asking you for info.
After you get Windows installed and running again -
- get the software patches that Dell marked as Critical. (may need to restart one or more times)
- get the Windows Updates and system packs ( if available, and start with critical only, also may require several restarts and re-checks )
- create a password reset disk
- install your antivirus software, and firewall if you don't use the one built-in.
... then start installing the optional updates and all your other software.
Now that you have done it, take all the disks and files, plus any printed or written info you have, and put it together in an envelope. If this is one of the desktop models, I usually just tape the envelope to the inside or outside of the removable door. Otherwise, file the paperwork and disks where you can find them - and they aren't going to get lost.