Your video card driver probably. Visit the web site of the manufacturer
of your video card, and download and install the latest version.

Pat wrote:
Sometimes when I scroll up or down on a worksheet in Excel 2007 the numbers
vanish from the sheet. If I click and drag to highlight the area where the
numbers are, they'll reappear, but this makes it very difficult to use my
spreadsheets efficiently.

Anyone have any ideas what could be causing this?