PC WILL NOT BOOT UP

I have a refurbished Dell 8400 Dimension PC [3.4 Gig processor] with Windows WP Pro preinstalled, that will not boot up. Will only boot at far as the WinXP logo, then blank screen apprears with cursor on it. Keyboard & other functions will not operate. No reinstall CD came with the system…that is why I’m trying to get far enough into system to retrieve my XP Pro Key. Where do I go from here, I am at a loss?

Gunny