KeyStateHelper.cs

// http://msdn.microsoft.com/en-us/library/ms646301.aspx
[DllImport("user32.dll")]
internal static extern short GetKeyState(int keyCode);

[DllImport("user32.dll")]
static extern void keybd_event(byte bVk, byte bScan, uint dwFlags, int dwExtraInfo);

internal static bool IsCapsLockOn()
{
    return (KeyStateHelper.GetKeyState((int)VirtualKey.CapitalLock)) != 0;
}

internal static void TurnCapsLockOn()
{
    // 0x14 is VK_CAPITAL
    const int KEYEVENTF_EXTENDEDKEY = 0x1;
    const int KEYEVENTF_KEYUP = 0x2;

    keybd_event((byte)VirtualKey.CapitalLock, 0x14, KEYEVENTF_EXTENDEDKEY, 0);
    keybd_event((byte)VirtualKey.CapitalLock, 0x14, KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP, 0);
}