I've implemented keyboard hooks in several languages (AutoIt, C#) using SetWindowsHookEx and WH_KEYBOARD_LL. I also know of a couple of C++ programs that have the same issue.
I didn't post any code because they work perfectly in Windows XP. However, under Windows 7, at some point the hooks become "unloaded" or stop processing any further keys. It seems like it may be related to a low memory condition, but I'm not really sure.
Did Microsoft change the way keyboard hooks work in Vista or 7 to add some logic that would unload third-party hooks under certain circumstances?
Related questions:
Well, it's been tinkered with plenty when UAC was implemented in Vista. Nevertheless, this is not a common complaint. Yes, it is quite possible for Windows to stop calling back the hook callback. A built-in feature to prevent the operating system from getting unresponsive when there is one hooker that doesn't handle the callback in a timely manner. It gets automatically removed from the callback list without any diagnostic.
This is based on a timeout and can indeed trip when the OS is starting to run low on resources. Like not having enough RAM and running lots of processes, getting massive paging. More likely with later versions of Windows since they need more RAM and tend to suffer when the machine was upgraded instead of wiped before the install due to disk fragmentation problems (especially the paging file).
The timeout setting can be tweaked by adding the HKCU\Control Panel\Desktop\LowLevelHooksTimeout value (DWORD, say 10000). Ask more questions about it at superuser.com