Do you also have any other 2142 mods installed?  If your 2142 is not 1.4 or if any of the other mods altered the original 2142 files in any way it'd screw up the MD5s, which would lead to punkbuster kicking you.  Usually though if that's the case it says you have modified content, though not always.

Also try to manually update your punkbuster as suggested on the previous page, and as straider said make sure punkbuster is checked in your options. 

Oh, and as a big one, press ctrl-alt-del and shut down any processes that are under your name on the Processes tab.  Or, at least, any that you can live without.  In ye olden days of 2142 I'd get the exact same thing if I had daemon tools or even my wordweb dictionary running in the background.  It sometimes (stupidly) trips on programs, so shutting them down could solve it.

Fear the Woodzilla