I have a logitech keyboard and mouse, both are wireless, but im sure the same problem applies with the wired version too.
in games such as counter strike using the volume, play/pause, next track, prev track, mute, media.... keys on the top interfeer with the games controls. for example, every tiome u hit pause, u drop your gun.
the way arrround this is to download the official software for the keyboard and mouse, run and install it. the problem then goes away, but the software doesnt half get annoying with all its popups...
dissable the program from startup by going into
start > run > msconfig > startup
remove the tick next to logitech iTouch
even though the program is dissabled, it no longer interfeers with other programs.