This problem is affecting all of my Excel worksheets that have controls
placed in them (e.g. Command Buttons). The controls have all disappeared!

Looking at their properties, it appears that the controls have all become
960 wide by 0.75 high, i.e. just a thin line. I can resize them to a normal
size, but there is no graphic displayed, only a blank rectangle with a red X
in the top left corner. The control still functions properly, e.g. it's
possible to click on a Command Button and get the Click procedure to run. If
I try to place a new control on the worksheet, it has the same problems.

The problem does not occur with forms (as opposed to worksheets). The
controls on forms all appear normally.

If I move these workbooks to another computer, everything appears as it
should be. So it must be a problem with my installation or configuration.
Excel has worked OK on this computer for years, this has just started
happening. Short of uninstalling and reinstalling Excel, is there a simple
fix to this problem?