warning Acidfree during mass import

Hello,

just after the mass import i'm getting the following warning:

warning: shell_exec() [function shell-exec()]: Unable to execute file -biN "c:/temp\tmp51D0.tmp" in D:\www.indydutch.nl\www.modules\acidfree\acidfree.module on line 2138

Has anyone else the same warning?
What is the best and correct way to deal with this?

Eddy