actually I have two more ideas for that
1) remove all floppy/zip/cd-r drives from the computers , that will diminish the risk of theft of files by taking them in floppies etc
2) get a software for computer monitoring, there are quite a few good packages out there , you will be surprised with what you find but depending on where you are you might have to make sure you state that you monitor the activity that is taking place in YOUR computer so they shouldnt expect privacy there