My son's computer won't boot. It is stuck on the "press tab to show post screen, del to enter setup". How do I fix this? Any help would be great.

What happens if you enter setup and select the internal hard drive (C drive) as first boot device?

When I turn on the computer this is the first screen I see. I doesn't give me the option to enter setup.

It is stuck on the "press tab to show post screen, del to enter setup"

You sure?

Yep, when I hit the Delete button nothing happens.

Would probably suggest a repair installation if that is an option for you

Repair Installation
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.

Perform a Repair Installation (http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx)

Who is the computer manufacturer? Do they have a diagnostic tool?

Sounds like your hard drive is fried and the system isn't recognizing it.