are you familiar with DOS - the command prompt?
How many files we talking here? 100s or 1,000s
If it were me I create a DOS Batch file.
Assuming all the files you want to change exit in the same folder.
Start -> Run -> "CMD"
Navigate to the required drive / folder - if you don't know how to do this you're gonna strugle with the rest ....
type "DIR *.* > change.bat" - this will create a file called "change.bat" that will include all of the files in that folder.
use notepad or other editor to amend the change.bat file - each line needs to start "REN" then the file name, then the new file name: eg "REN file.ext newfile.ext"
You'll need one line for each file to change.
Stip out all the other header and footer lines - as not required
Save the file change.bat - then double click it - volia!