How do I close the Appearance Preference Window that keeps opening when I try and open an excel file

If you run the spreadsheet program that you want to use to open the file (I am guessing this is LibreOffice Math, or OpenOffice.org Math if you're using an Ubuntu release prior to 11.04 Natty), can you open the file from there?

If not, then we know the problem is probably with LibreOffice (or whatever other program you are using). If so, then we know the problem is with your file associations (which, from your description, seems more likely).

If the problem is with your file associations then you can usually fix it by right-clicking on the spreadsheet file in Nautilus (your file browser) and using Open With... (or Open With Other Application...) to set LibreOffice (or whatever program you want to use to open that kind of spreadsheet file) as the default.

If the problem is with your file associations and you are running Ubuntu 11.10 Oneiric Ocelot and you cannot set the default application you want using the above method, then see http://www.techdrivein.com/2011/10/does-new-ubuntu-1110-prevent-you-from.html.

If the problem is with the spreadsheet app itself, or for any other reason the above information does not enable you to solve the problem, then please post a reply so we can keep working on it. (If it does enable you to solve the problem, then you can mark this question as Solved. One way to do that is at https://answers.launchpad.net/ubuntu/+source/software-center/+question/175880.)