Matt,
I use GraphicConverter by Lemke Software.  It can decompress anything
I have run into, and can convert a large number of formats.  Sorry, I
am not personally familiar with .art formatted files, so don't know
if it can handle them specifically.  I use a Mac version of the
software, but I believe the .exe version is also available.  The
intro screen of the software lists a website at www.lemkesoft.de and
www.lemkesoft.pingnet.de
 
Regards,
Paul M.