Painkiller Overdose multiplayer was tied to GameSpy master servers. When GameSpy was shut down, the open source community created their own master servers called OpenSpy. These fixed DLL's now refer to openspy.net instead of gamespy.com.
Installation:
Open the folder with the version of Overdose you have installed: v66.2 (demo), v75.3, or the latest v84.4.
Extract the required DLL files to C:\Program Files\DreamCatcher\Painkiller Overdose\
The Backup folder contains the original unchanged files from the game.