It is quite easy and very useful if you are running complex and long calculations.

1. Add Excel to you Start Menu. – In the program files right click Excel and chose “Add to Start Menu”
2. Shift Click Excel on Start Menu – this opens a new version separate from any previous one.
3. Add Excel to you Taskbar. – In the program files right click Excel and chose “Add to taskbar”
4. Shift Click Excel on the Taskbar – this opens a new version separate from any previous one.

As far as I can tell you can only run two versions at a time.