Wordpad documents default setting is to save files as .rtf files (Rich Text Files)...In Windows Explorer, if you originally had the settings to "Hide file extensions of known file type", then the files show no file extension..That's why you originally saw only the file name but no file extension...When you changed the Windows Explorer settings it now shows file extensions..That's completely normal..
In order to check, assuming you're still using your XP operating system, double click on "My Computer". When it opens, click on "Tools" in upper left, choose "Folder Options". When that opens, scroll down to make sure the box next to "Hide extensions for known file types" is UNCHECKED. If it is, then everything is just as it should be. To change it back to NOT show file extensions, place a CHECK mark in the box, then click on "Apply", then OK.
Hope this helps.