AutoCAD 2010 :: Block Geometry Disappears

Just recently we upgraded from ACAD 2010 to 2011. Some users are still on XP and some are on Windows 7. Since we began making the jump to ACAD 2011, we have begun to notice more and more drawings popping up with blocks no longer visible that were there the day before. When you go to the block editor, the block definition is still there but all of the geometry has been blown away. The only entity that seems to be immune is an attribute. The only commonality among the various drawings this is happening too is that they are all being accessed from an XP machine. Some folks have had to spend hours restoring all the blocks that were lost and that in itself was a challenge because ACAD already saw the definition so it would continue to blow away the geometry. You could purge the block IF you could find a way to pick it and delete it.