Some users running Microsoft Security Essentials experience a restriction kick when playing on PunkBuster enabled servers: Corrupted Packet Flow.
This will show you how to be able to play although Microsoft Security Essentials is running.
First right-click the Microsoft Security Essentials icon in your taskbar and select “Open”. You will see this screen:
Then goto the “Settings” tab and choose “Excluded files & locations“. Click the “Add…” button and a file chooser will appear. Find the files “PnkBstrA.exe” and “PnkBstrB.exe” in your Windows’ System folder. Under Windows 7 (64bit) it will look like this:
Keep in mind that the path for 32bit Windows editions is different. The 64bit System folder is SysWOW64 while the 32bit one is System32.
Save the changes and you are done.