what situation is I can imagine being the cause is that the .DLL is detected due to having some kind of unwanted program (not virus), now that on the only PC that the .DLL gets deleted Virusscan has Antispyware module installed and on the other two it has not.
When the antispyware module is not installed, detection might occur, but no action will be taken.
Also please try scanning the file from the other two workstations (like in a shared folder, etc.), too, see if they detect anything in it (not in their own copy locally).
Also, could you share with us the name of the detection?