If you're looking to add a suffix or prefix to a bunch of files in Windows, here's a quick PowerShell command for doing that to all files in the current directory:

dir | Rename-Item -NewName {$_.BaseName + "_suffix.txt"}

Note that using BaseName omits the file's extension, so you'll need to add it yourself. If you don't want to omit the extension, use Name instead.

Original Stackoverflow answer can be found here. That also covers filtering files, so check it out if that fits your needs.