だーかーらー
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/userinput/keyboardinput/keyboardinputreference/keyboardinputmessages/wm_keydown.asp

lParam
Specifies the repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag, as shown in the following table.
<中略>
30
Specifies the previous key state. The value is 1 if the key is down before the message is sent, or it is zero if the key is up.