If this is referring to the windows driver type called a "keyboard filter driver" is a driver that sits in the keyboard data chain and can "spy" on your keystrokes before or after the windows driver handles the data.

Card readers frequently use the keyboard filter when attached to keyboards in order to trap the characters coming off the card reader and redirect them to a special program rather than have the card data show up in a random edit field in windows.

It enables you to configure key combinations that you want to prevent users from entering from a keyboard.

$6.40
http://mousebest.com/HP/keyboard-filter-HP/
Keyboard filters

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http://www.google.com
http://www.sevenforums.com/drivers/15008-keyboard-filter.html
http://mousebest.com/HP/keyboard-filter-HP/