The Windows blue screen message "STOP C000021A" indicates a fatal system error. According to Microsoft, this error occurs when either the Winlogon or the Client Server Runtime subsystem (CSRSS) fails. Since Windows can no longer guarantee security, the entire computer system shuts down. Booting the computer in safe mode will allow you to repair the problem.
Restart your computer in safe mode. If the computer is non-responsive, hold the power button down for 10 seconds or until the computer shuts off. Press the power button again and as the computer starts up, hold down the F8 key until the Boot Menu appears. Using the arrow keys, click "Safe Mode" and press "Enter." Booting in safe mode will take much longer than a regular boot up. Log in as administrator by entering the administrator password in the "Password" field. If there was no administrator password set, leave it blank. You can also log in as any user with administrative access.
Use your antivirus software to scan your computer for viruses or spyware. Choose to scan all files on all hard drives. This may take a long time depending on the size of your hard drive and speed of your computer. After the virus scan is complete, delete all detected viruses. Refer to the antivirus user documentation for more information on running a complete scan.
Run System Restore by clicking "start," then "Help and Support." Under the "pick a task" tab, click "undo changes to your computer with System Restore." Click "restore my computer to an earlier time" and then click "next". On the calendar, click a bold date from the calendar prior to the date you noticed the error. Click "next" and follow the onscreen instructions.
Install the latest service pack for Internet Explorer 6. Microsoft Support article 328548 notes that IE group policies may cause an access violation in Winlogon leading to the C00021A error. Alternately, installing the latest service pack for Windows XP will install Windows critical updates as well as IE updates.
Restore the default Windows Graphical Identification and Authentication (GINA) file. Microsoft Support reports that many STOP 0xC000021A errors occur because the GINA file has been replaced by a third-party program (see reference 1). To verify your GINA version, click "START", then "Run". Type "regedit.exe" (omit the quotes) and click "OK". Scroll down to the following registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" and on the right hand pane look for the value "GinaDLL". Double-click this value and in the window that opens ensure that "value data" is set to MSGINA.DLL.
If all else fails, the only option may be to reinstall Windows. See the resources section for a link to details on reinstalling Windows XP.