using System.Runtime.InteropServices;
...
        [DllImport("user32.dll")]
        static extern void keybd_event(byte bVk, byte bScan, uint dwFlags,
        UIntPtr dwExtraInfo);
        const int KEYEVENTF_EXTENDEDKEY = 0x1;
        const int KEYEVENTF_KEYUP = 0x2;
        const int CAPSLOCK = 0x14;
        const int NUMLOCK = 0x90;
        const int SCROLLLOCK = 0x91;

keybd_event(NUMLOCK, 0x45, KEYEVENTF_EXTENDEDKEY, (UIntPtr)0);
keybd_event(NUMLOCK, 0x45, KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP,(UIntPtr)0);