oh dear god... this is why people should take a test before being given access to the net.
ok in explorer go to the Tools menu and select folder options
select the views tab and then uncheck the box for "Hide extensions for known file types"
and then you should be able to change the extension of the file, by clicking on it and pressing F2.
also it's a good idea to show extensions as there was (may still be for some people who don't update) an exploit where XP would show you the file was associated with one program but would run in another as the file would have an extension like word.doc.exe and it would appear to be a word document, causing you to run a potentially malicious file.