Some versions of Excel are able to repair themselves. Make use of them to the fullest extent possible.
If you are running Excel 2003, from within Excel select Help | Detect and Repair... from Excel's main menu. Follow all instructions from that point.
If you are running Excel 2008, click on the Office symbol at the upper-left corner of Excel's main window:
You should see a window like this:
In the left-hand panel, click Resources. The right-hand panel should change to this:
In the right-hand panel, click on run Microsoft Office Diagnostics, then follow all instructions. If this does not work, go to the next step.