There should be proper preview icons for all library parts on all platforms, all the time, no excuses. This has been a long standing problem on the Mac side and now it seems to be an issue with 64bit Windows (archicad-talk.graphisoft.com/viewtopic.php?t=31958).
Since I can create my own Mac library part icons by simply copying and pasting into the Finder Info panel it seems like this should not be too hard to do.