I have ntdll.dll version 5.1.2600.1106. After download ntdll.dll version 5.1.2600.2180, I can’t paste new version, because old file already in use.

The 2180 should be in SP2. Do you have SP2 installed?

The typical replacement of such files follows this scenerio:

Set the BIOS to boot CD before Hard Disk. Boot the XP CD and, instead
of Setup, take the immediate R for Repair. Assume any password
requested is blank, and TAB over.

If the CD drive letter is X: use
Copy x:\i386\ntdll C:\windows\system32


But if you have installed a Service pack (eg SP2) do the copy from
C:\windows\ServicePackFiles\i3*86


not the CD