Dell might have a recovery partition...so you may want to wait for some more knowledgeable posters to reply before trying my method, but if you reformatted it may be gone.

 

My Method below will most likely mean all of your old data is gone.

First put your Windows Installation CD or Dell Recovery CD in your CD Drive

I am guessing that DOS like screen you are seeing is you BIOS...there should be a option in there for BOOT SEQUENCE.  If you do not see this you may not be in BIOS..when you start your computer and see the Dell logo (I assume you have a Dell) and the blue bar going across, look in the upper right corner and it should say hit a certain key to enter SETUP...it should be "F2".  Hit that key.  Then arrow down to BOOT SEQUENCE. Then hit enter...arrrow down to CD/DVD Drive and hit enter.  Find option that says "Save changes and Exit".  This should restart your computer, if it doesn't turn it off and turn it back on...The Computer should boot from the Install disc and you can follow the prompts.  You may be given the option to recover a version of Windows XP...again I don't know if this will work if you reformated your hard drive.

Depending on how far he's gone, PC Restore might not be an option. From the sound of it, he's gone that far and altered the MBR. Follow HOOfan_1 instruction to give this another shot.