Ok, here it is.
OS is Vista Enterprise
The problem happens with several applications, mostly with word 2007. every log says the fault is ntdll.dll
The file is present in C:\windows\system32\
Any effort to change the fila with a new one tells me I have no rights (with both local admin account and enterprise domain admin account).
Trying to register ntdll.dll results with error message that dll or ocx file is missing or corrupt.
sfc /scannow in cmd gives no files with errors.
Any idea how to solve this?