If you are playing Battlefield Bad Company 2 on PC through Steam, you may be getting kicked by Punkbuster after a few seconds anytime you join an online game. This is most likely due to a conflict with Punkbuster and the BFBC2 Beta installation on your PC.
To fix this:
1. Go to C:\Program Files (x86)\Steam\steamapps\common
2. Delete the Battlefield Bad Company 2 Beta folder, which includes Punkbuster
3. Go to Punkbuster's website and get www.punkbuster.com/index.php?page=pbsetup.php
4. Launch the program
5. Once launched, click add game
6. Select BFBC2
7. Browse to C:\Program Files (x86)\Steam\steamapps\common\battlefield
8. Click OK and then click Check for Updates.
Punkbuster will now initiate a download for the BC2 Punkbuster client.
UPDATE: You may also need to remove the BFBC2 Beta folder from "My Documents" as well before this will work properly.
NOTE: Do not let Punkbuster run in it's default destination. It is set up to go to the BFBC2 Beta folder instead of the correct BFBC2 release version folder.