Try this:
1. Open MS Excel 2003 from the start menu.
2. Go to Help > Detect and Repair
3. Start the repair and wait approx 5-10 mins.
When double clicking on .XLS / .DOC files within Windows, they should now open with Office 2003, instead of Office 2010.
However if you open something in Office 2010 again, it may become the new default.
Hope this helps!