Step 1
Download these files:
http://www.evenbalance.com/downloads/pbsvc/pbsvc.exe

Put them in a folder named "PunkBuster".

Step 2
PunkBuster service

Run pbsvc.exe as administrator,
Select "Install/Re-Install PunkBuster Service", select "Next",
Select "I Agree",
Wait then select "Next",
Wait again and finally select "OK" and then "Finish".

Step 3
Setting the Stage

Extract the archive "pbsetup.zip" and be sure to have Notepad++ or equivalent (we will use it to edit "pbgame.htm").

Step 4
A bit of programmation!

Right click on "pbgame.htm" and select "Edit with Notepad++",
Add the following lines as on the info:

<p> game cod2 "Call of Duty 2"
<p> game_os cod2 w
<p> game_hint w cod2 "C:\Program Files\Activision\Call of Duty 2"
<p> game_hint w cod2 "C:\Program Files (x86)\Activision\Call of Duty 2"
<p> game_hint w cod2 "C:\Program Files\Steam\SteamApps\common\Call of Duty 2"
<p> game_hint w cod2 "C:\Program Files (x86)\Steam\SteamApps\common\Call of Duty 2"


Step 5
Get the PunkBuster files for Call of Duty 2.

Run pbsetup.exe as administrator,
Select "I Agree",
Select "Add a Game" then in the game list select "Call of Duty 2",
Be sure the game path is ok - if not good, click "Browse" and navigate to the installation directory of the game and select it - then select "Add Game",
Wait even if PunkBuster says he doesn't respond!,
And finally click "OK" then "Quit".
from: Steam.com