Excel

Whenever I try to edit an existing worksheet, by selecting the active cell with the mouse, it will not allow any info to be entered into the cell and I cannot do anything because the cursor will not change back to the e\arrow to access the menu commands. It selects everything in its path. I have tried clicking the mouse etc. always with the same result. I have removed and reloaded office 2000 and even loaded office xp, no change!
Please help!!!! thank you!