@Mkz:
I know which defragmentation method is used, since it is the only method availabe on WinNT (see above). Opening the file for reading is only required if the file is EFS encrypted. Else plain "read attribute" access is needed, like you wrote. Since reading attributes is even possible from Windows Explorer, the defragmentation program will most likely be able to get the same file access.

@Lunar_Dust:
I found a programm called "File Lock" (yes, plain name ), which is able to lock and/or hide files or directories. I tried it with O&O Defrag, Diskeeper and PerfectDisk.

The O&O Defrag service simply crashes completely when analysing a volume with a locked file (you have to reboot before the GUI can connect to the service again).

PerfectDisk simply ignores the fact that the file ist locked an defragments it, even if the file is also hidden.

Diskeeper does not defrag a locked file, finally. I will test it tonight on our server, tomorrow I can tell if it worked.

@all:
Thank you all for your ideas.