The file extension XLS is a proprietary file format of the Microsoft Excel spreadsheet program. The .xls format is the standard format for versions 97 through 2003. As of version 2007, Microsoft has released a new version of the Excel Spreadsheet format.
An XLS file contains table cell values as well as formatting instructions, formulas, and possibly macros. These macros may also contain malicious code, which is why it is advisable, especially for XLS files from unknown sources, to subject them to virus scanning before executing the macros.
XLS is based on the proprietary binary format of Microsoft, the Binary Interchange File Format (BIFF) format, which, however, can still be read by almost any spreadsheet program due to its age and its high popularity.
Most often resolving problems with opening XLS files is very simple. Just install an appropriate program that supports such files. All of the listed programs support XLS files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of XLS files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate XLS files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Microsoft Excel applications is not on the list, select "Browse" option in order to manually locate the directory where Microsoft Excel has been installed.
Sometimes problems with opening XLS files may not lie with the application itself, but can arise due to other causes, such as: