You really need to know the three letter extension to the file ie filename.xxx. The operating system makes the decision as to which installed applications open with what file extension (eg filename.doc opens with Word or filename.pdf opens with Adobe Acrobat). The problem is that by default Windows hides these file extensions. You can either make them visible (Right click "My Computer" then left click "explore" > Tools > folder options... > In the "View" tab ensure "Hide files extensions for known file" types is UNticked. Alternatively, right click the file in question > properties will tell you the entire the file.extension name. Tell us what the offending extension is and we will tell you what application you need to view it.