How to change file extensions of multiple files in XP?

You could use the command line interface and use the rename function:

C:\folder\> ren *.* *.newextension

This will rename everything to ".newextension".

You could use Massive File Renamer

It will allow you to easily rename multiple file extensions. It's very fast and simple!

For advanced users and developers, it is possible to use regular expressions. See it in action: