Hi Deano,
If you have any "Excel" documents on your desktop that have not downloaded correctly, then right click on the downloaded document, (ctrl click if using single button mouse) and choose "get info."
When the get info screen appears, just over half way down under "name and extension," you will see the words "Open with" with a small black arrow next to it on the left hand side.
If you cannot see the name of the application listed under "open with" click on that little arrow. Now, you will see the name of the application your system is using to open these type of documents.
You need to change it if it is not listed as "Microsoft Excel" To do this, just click the arrows next to the name of the application listed and bring up the drop down menu, and click on other...
Find the actual Excel application on your system, which should be in your applications folder under Microsoft Office and click on that.
Before you close the "get info" screen, click on "change all" button within the get info screen. Your document should now open as an Excel file, if it does not, restart your system, because sometimes I find this is necessary for the change to take effect.
