At startup the Windows XP logon screen does not appear and the computer continuously restarts
The problem can occur in Windows XP Home and Professional Editions.
“SYMPTOMS: When you start your computer, the Microsoft Windows XP startup screen appears, and then the computer may restart. The Windows XP logon screen does not appear.
“CAUSE: This issue may occur if the Kernel32.dll file is missing or damaged.
“RESOLUTION: To resolve this issue, use the Windows Recovery Console to extract a new copy of the Kernel32.dll file from the original Windows XP CD.”
Note that when you make changes to system files always make sure that you have a recent system backup or system image that you can restore should anything go wrong that prevents Windows from booting, etc.