What type of file is it? Most files have a file name with a three or four letter extension which tells the machine what program it belongs to. For example, a .XLSX file is a Microsoft Excel spreadsheet file. This information is stored on your Windows machine in a table of File Associations. In most cases, the File Associations table is updated when you install a new program, and your PC knows what files each program can handle. But sometimes, a new data file can fool it, or the program has been deleted.
Note: If the file does not have an extension on the file name the machine will have trouble identifying it. If you know what program created it, you can rename the file to have the correct extension. However, just adding a file extension will not convert the actual data from one type of file to another (Putting a .XLS on the end of a .DOC word processing file will not turn the letter into a spreadsheet.)
If the file was made by a program that you don’t own on your PC, then Windows won’t know what to open. Or, if your file association table is sending the documents to a program that you don’t want to use, you’ll want to correct that.
If it is a file type that you know you can open with one of your existing programs, then you can Right click on the file icon, and choose Open With… and choose the program that you want to open this file with. If your desired program doesn’t show up on the list, click Choose Default Program… and Browse… to find it.
If you want all files of this type to open this program in the future, after you have clicked Choose Default Program and found the program, click the box that says “Always open files of this type” and it will make that association permanent. http://support.microsoft.com/kb/307859
If you don’t know what program made the file, or if it came from a Mac or Linux system, you may need to do some extra research to find out what software is capable of opening the file. Preview will open most graphic files, Photoshop can open files from more types of graphic programs, Word (or OpenOffice) can open many different word processing file types, Excel (or OpenOffice) can open spreadsheets plus many text-based data table and database types such a .csv, .tab, .sylk, .wks and others.
FileInfo has lists of common file extensions and their programs. www.fileinfo.com/filetypes/common
If your file association table has become damaged, and all of the documents are not opening correctly, you may need to replace it.
In Windows 7, To make a particular program set itself as the default for all the file types it can handle, or to edit the list of extensions it is set for, go to Control Panel, Default Programs, Set Default Programs, and choose the program from the list. Then choose Set as Default or Choose Defaults for this Program.
In Windows 7, to edit a particular file extension’s default, go to Control Panel, Default Programs, Associate a File Type or Protocol with a Specific Program, click on the extension in the list, and click the Change Program button.
In Windows XP, go to Control Panel, Folder Options, File Types, and either click New to create a new extension, or click on an extension in the list to edit it.
There are also some third party programs that can help edit file type associations
Types izt.name/soft/types/
This free program will repair the most common associations in a damaged Registry www.thewindowsclub.com/file-association-fixer-for-windows-7-vista-released