If you are a Windows XP user and if you are required to change the extension of a file, then your chances of succeding are very low unless you have some tech knowledge
Extension is the 3 letter term used after the dot in a filename like .exe for executable files .txt for text files .doc for office documents .xls for spread sheets, it may sometimes be just two letters long like .pl for Perl language files. By default windows doesnot show you the extension of a file, only those files which it are not yet associated with any program are shown along with their extensions. Rest of the common file types dont have extensions displayed, when you try to change a file’s extension when its extension is not displayed you will just end up adding the new extension to the filename and not to the extension part of it, like you have a spreadsheet named accounts which is actually accounts.xls and you want to change it to a text file called accounts.txt when you just rename it you will end up renaming it to accounts.txt.xls where accounts.txt is the filename part and .xls is the extension part. So the solution for this is to make windows display the extensions of all files, to do this open Windows Explorer or commonly known as the My Computer and go to the Tools Menu (Alt + t) and then click Folder Options, in the dialog box change to the View tab and look for Hide extensions for known files and uncheck it and click OK.
Now all your files have extensions displayed along with filenames so when you want to change a file extension you can just change the 3 letter part after the . (dot) in the filename. One thing you should take note of is when you want to just change the filename make sure you dont alter the part after the . in the filename. You have to change only the part before the . to change the filename.
If you feel strange having all the files with those 3 letter extensions you can change it back to normal by checking the Hide extensions for known files option in the Folder Options dialog box.