Virtually every time I see a person who is having trouble with a new computer, most of the confusion stems from the fact that, by default, Microsoft does not show complete file names in the folder listings. It’s supposed to make things easier but it doesn’t because you aren’t getting to see the actual file name. I’ve talked about this in earlier versions of Windows, but the problem continues with Windows XP.
On any Windows computer, whether you realize it or not, the files ALL have an "extension." The extension is usually three letters and appears after a period at the end of the file name. You may see "My Really Cool Report" as the file name, but your computer knows it as "My Really Cool Report.doc". The .doc at the end tells the computer that it is a Microsoft Word file. Every program appends an extension at the end of the file name.
The file extension (such as .doc, .exe, .psd, and so on) indicates what type of file it is. Windows uses the file extension to figure out what software program to run when you double-click the file. So when you double-click My Really Cool Report, Windows knows it’s really My Really Cool Report.doc, runs Microsoft Word, and opens your file.
In Windows parlance, Word is "associated" with the .doc extension. Frequently, the extension stands for something, so they’re easy to remember. For example, .doc stands for document, .bmp stands for bitmap, and .ico stands for icon.
Windows needs to know about file extensions, but so do you. In order to actually understand what you are looking at, you need to tell Windows to show the extensions. Someday you will get a file that isn’t associated with a program on your system or want to import it into another program. You can’t easily work with a file, if you don’t even know what kind of file it is.
To show file extensions in XP, go into My Computer or Windows Explorer and choose Tools|Folder Options. In the View tab, remove the checkmark next to Hide extensions for known file types. And if you prefer the way Windows 98 folders worked, here’s your chance to change XP. In the General tab, click Use Windows Classic folders.