'Restoring' to factory settings - Windows XP

I have a Dell Latitude D620 running Windows XP, the same OS as when it was new. I'd like to restore the computer to factory settings or get the OS as close to 'stock' as possible. However, I do not have a product key or an instillation CD, because the computer had the OS preinstalled. I have tried to boot computer from an earlier time, but it says there are no boot points. I'm not sure if there is a way to restore it given the conditions, but any help is appreciated. And if all else fails are there any downloadable 'programs' that would delete all unnecessary files not associated with the OS (like a contacts cleaner that deletes all the extra contacts but instead of deleting all the extra contacts, it deletes all unnecessary files to get a near 'stock' OS) , I'm not sure if this exists, or if they are even safe for the computer, but I am young and very new to Windows so thank you for all your help I very much appreciate it.