Ausatco,
Ntdll is a core component of WinXP. Here are the details on the file. Make sure yours matches:
22-May-2003 21:46 5.1.2600.1221 654,336 Ntdll.dll with SP1 (Time is in UTC.)
You could try replacing the file. If that does not do the trick, you could try a Repair with the WinXP CD before you do a clean install of WinXP on top of itself.
Take Care,
Richard