altering .txt files to .xls files

I have a file from 2007 that I need to convert into excel format. I already tried renaming the file to .csv and .xls and that did not work. Can anyone help me? I need this conversion due to a customer reqesting back up data for an invoice from Jan 07. Currently running windows Xp.

What type of data do you have in you file? Simply renaming a flat text file to csv will not allow excel to read it.