can't boot to an operating system

Tom had called into our show to say he had been unable to boot to Normal mode, Safe mode, and a command prompt.

Advanced troubleshooting for general startup problems in Windows XP
http://support.microsoft.com/kb/308041/

He might be able to boot into the Recovery Console...

Try to use System Restore...

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He might need to try to do a "repair installation":

How to perform a repair installation of Windows XP if Internet Explorer 7 is installed
http://support.microsoft.com/kb/917964/

How to perform an in-place upgrade (reinstallation) of Windows XP
http://support.microsoft.com/kb/315341

If you perform an in-place upgrade of Windows XP, all existing restore points are removed and a new system checkpoint restore point is created after the in-place upgrade is complete. Do not perform an in-place upgrade if you may have to use System Restore to restore your computer to a previous state.

You may lose data or program settings after reinstalling, repairing, or upgrading Windows XP.

If you have Windows XP Service Pack 2 installed on your computer, you must reinstall Service Pack 2 after you reinstall or repair Windows XP...

After you perform an in-place upgrade or repair installation, you must reinstall all updates to Windows...