Clicking from one cell to another causes a range

When I click from one cell to another, it causes a range of cells to be highlighted. I cannot enter any data into a single cell. For example, on a blank sheet if I type into cell A1 it causes a range to C1. Then I cannot get to B1 to type anything. If I click to another cell it just makes the range bigger. What can I do to fix this problem?

Hi,

No, but strangely enough the problem seems to have fixed itself. I have no clue - I must have hit some strange combination of keys or clicked on something by mistake. Thanks anyway - I'll remember that for next time.