How do i restore my dell to factory settings?

Hello,

i have a dell dimension 5100 with windows xp on it, i'm trying to reset to factory settings after backing up necessary files onto an external hard drive. I pressed f8 when starting the computer, but my option was to return computer to state when last worked. I actually want the factory settings. the computer works, but is so slow I want to throw it out the window. can you tell me how to get factory settings?

thanks