Many users (including customers of mine) have experienced trouble with two recent security updates by Microsoft, see:
There seem to be two hotfixes to this problem:
KB973475 Excel 2003 hotfix package :
KB973593 Excel 2007 hotfix package :
If you experience problems with your Excel, check if you have this update and if so, download the hotfix.
For other problems related to starting and stopping Excel, see:
Jan Karel Pieterse