How do you "validate" game files?

[Afterburn3r]

my DOD:Source isn't working, and someone told me that i should "validate" my game files... how would i go about doing that?

 

[Ames]

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=282