how can I return to dell factory installed operating system?

Reinstalling OS will break CTRL + F11 functionality. If you didn't delete the recovery partition (the data is intact but not accessible), you might still be able to recover the image files. Take a look at the following thread:

http://forums.us.dell.com/supportforums/board/message?board.id=sw_winxp&message.id=150074

particularly Dan Goodell response on page 1.

 

The idea is to change the recovery partition type so you can access the data from within Windows. Once accessible, you'll be able to copy the image files to another location, then make yourself a bootable DVD so you can boot off DVD and run recovery.exe to recover your factory installed OS. Hope that helps.