Excel Button and Drop down image missing
 

I have a excel workbook with a couple of buttons and drop downs. At
some random instances, the button image disappears and there is a red
cross in their place (Similar to a what you get if a image is missing
on a HTML page). The button seems to work fine if I click on it.

The combobox does something similar. The image is missing. But if I
click on it, the screen fills up with 1 giant word (which is the
current selection). In other words, the height of the combo box becomes
the screen height. If I scroll down(by the arrow key) and press enter,
the Excel page is visible again.

The problem seems to disappear if I restart my computer. Is this an
issue with my computer or is it something to do with Excel.

Thanks in advance,
Bharat