This error usually comes up when there is a formatting error in the list (csv/txt) file. We usually see this when there are delimiters in the data column for the file - when commas are within a cell in your .csv file or if tabs are within a cell in a .txt file. The most direct fix would be to delete any unnecessary columns from your file prior to import.
Your file must contain email addresses but all other columns are optional and only useful if you intend to mail merge or segment new lists with the data. For example, columns with Street Address data often have commas in the data and few customers would find it useful to include a customer's street address in an email or to segment a list on this data.
If you have data entries that have commas in the data entries and you want to separate them into separate columns, you can do so using the text-to-columns tool on Excel's data menu.You can learn more about this tool here: http://support.microsoft.com/kb/214261