What operating system?  Windows XP or Vista?  If it is Win XP you should be able to use the Dell restore partition.  Restart the computer and press CTRL and F11 when you see the F2 and F12 menu appear at the upper right of the screen.  Release both at the same time before the blue bar disappears that shows up just before Windows starts to load.  If you don't see this, tap the keys as soon as you see the first display as the computer begins the boot process.  If successful you should get a menu asking if you wish to restore the computer and warning you that any data or programs you have put on the computer since you got it will be completely erased.

 

If you have Vista you need to ask this question in the Vista forum.  I believe Vista has a restore process you access from the menu you get when you press F8 as Windows begins to load.

 

Edited to correct an error due to trying to do too many things at once.  Thanks for posting Bev, don't know what I was thinking.