windows - Why is SetWindowsHookEx() returning NULL and LastError is ERROR_HOOK_NOT_INSTALLED -

i have code has been working without reported issues few years. sets low level keyboard hook user uses activate application.

m_hkeyboardhook = setwindowshookex(wh_keyboard_ll, keybhookfn, getmodulehandle(null), 0); 

for 1 user of sudden (it worked him before years well), keyboard hook isn't working. upon investigation, can see setwindowshookex() returning null , getlasterror() returning 0x00000597, error_hook_not_installed.

i can't find documentation on means.

what possibilities why might happen? system security settings? user account privileges? user using windows 7.

i need specific answer how make error occur can solve user's problem

looking @ http://blogs.msdn.com/b/alejacma/archive/2010/10/14/global-hooks-getting-lost-on-windows-7.aspx, seems might occur if cpu load high.

another thought: did break when upgrading 64-bit windows? don't know if applies ll hooks, cannot inject 32-bit dll in 64-bit process.

finally: ll hooks won't capture input in uac-elevated process if hook not installed uac-elevated process (a lower privilege level process cannot capture input of higher privilege level one)