Excel "has stopped working" (ie. crashes) on closing programme
 

Excel 2007 version

Sometimes unable to open Excel and receive message "Microsoft Excel has
stopped working" followed by crashing of programme before opening fully.

Able to work and save work ok once Excel does open

On attempting to close Excel programme ALWAYS get error message "Microsoft
Excel has stopped working" followed by crashing of programme.

Any tips as to why this happens and how to stop it?