Some operating systems hide file extensions by default. I assume this is your problem.
These instructions are to fix it for windows 7, but by memory XP is similar.
1) Open a folder (eg .minecraft/bin)
2) Click 'tools' from the menu above (if no menu is visible you need to press alt first to make it show up)
3) Click 'folder options'
4) Click the 'view' tab
5) Untick 'Hide extensions for known file types'.
6) Apply and hit ok.
From now on all files will show their filetype in the name. Minecraft will be Minecraft.jar, a music file will be SomeSong.mp3 instead of just SomeSong etc.
From now on whenever you rightclick on any file and click rename you will be able to change both the files name AND the extension type (eg .jar to .zip).
If you are going to keep file extensions shown in general be careful. Make sure when you rename a file you don't change or remove the extension by accient. For example if you try to rename a mp3 and you revmove the '.mp3' from the end windows will no longer know it is a music file (until you rename and put the .mp3 back on the end). Generally if you do change the extension name tho, windows will warn you first so just keep an eye out for this warning message.
I have the same problem... Kind of... I have a mac and suddenly ALL of my minecraft files (.jar) have changed to .rar (have the file that says RAR on it)... I uninstalled minecraft and re-downloaded it and all of the files are still .rar Does anyone know how to fix this? I can't install mods because of it... Can someone help?
(Sorry this is on this post/feed/thing... I dont know how to post my own post)