XP is reaching End of Life and reinstallation of Windows XP should be avoided.
As your OS cannot boot follow the section on Windows Reinstallation Guide/F12 Pre-Boot diagnostics (page 374)
If diagnostics pass move onto clean Windows Installation. You should consider upgrading to a modern Windows OS as mentioned but there are instructions in the guide for installation of XP also.
Note as the system is getting pretty old it may be better to purchasing a new Dell Desktop or purchasing a second hand OptiPlex 760 which has a Windows 7 license opposed to buying a Windows license separately with some possible upgrades.