Some people have encountered this issue. I know that Zoopy had a problem where he could not switch to his secondary weapon or switch weapons using his mouse if he pressed a certain combination of buttons on his mouse (or something along those lines)

Steam thread

http://forums.steampowered.com/forum...d.php?t=787871

Technical tutorial:

http://www.nvnews.net/vbulletin/showthread.php?t=91724