cannot edit a dimension

I cannot edit any sketch dimension in a new or old file.
I get a blank or unchanged dialog box when editing, AND two
new dialog boxes come behind. One is related to Shared and the
other is Linked.
I'm using the student design kit 2008-2009. It has been workingfine since January.
Now I cannot create any new sketch or change any sketch dimensionI've done since January. So this is a global issue. How do I getout of this?SolidworksSketching