Can't delete JPG files

After having manipulated JPG files (opened, moved, copied,...) many
(only JPGs - and not all of them) cannot be deleted/moved anymore. An
error message says "Access violation. The file might be open." (From my
user point of view it is not, though.)

This seems to have to do with the application ACD mPowerTools (when
uninstalled, above effects have vanished, too). Apart from trying to
find out, if this is a bug in mPowerTools, I'm interested in learning
what actually might have happened to such a file. Is there a way to find
out, if another program got a hold on it, and which program if at all?

btw: The files concerned are not write-protected, and don't seem to
have any specific characteristic (apart from being JPG, of course).

Any ideas???

Txs, Peter