Microsoft complaint of the year:

Somebody at MS-land has an agenda that may have been well intentioned to simplify things for for totally ignorant users, but it actually hurts the average user. Bill must want future users look at icons instead of extensions for file types. The power user changes this right off, so is just an irritation. It is a headache on phone support. MS hides extensions to the file name as in .txt for a text file, .doc for a word file, .xls for a spreadsheet. These were essential in DOS and early windows versions for applications and the system to recognize their own associated files, and for the system to open the right application if the file is called on first. These extensions are still used extensively by all the system and programs, and users need to see them to operate beyond toaster level. However, since Windows ME forward, systems come with extensions hidden by default. The icon method? Who knows what those cryptic things are anyhow? They can sometimes be a good visual clue, but should not replace having all the information you need to control your computer.

Go to tools|folder options|view and uncheck Hide extensions for known file types to fix. While you are in there you might want to check the Show hidden files and folders, especially handy if you are trying to find and delete a virus file, but the operating system is slowing you down by hiding it from you.