My Vista x64 computer won't go to sleep while idling with any setting above 15min! It does it just fine with 15min setting and anything below it, but I need to make it sleep after 45min of inactivity. The screen turns off perfectly fine, but computer keeps running after time runs out!
I also thought that Security Task Manager software from Microsoft that let's you block keyboard, mouse and other activity monitoring (like key and activity logger) could affect that, but when I turn the monitoring back on, it doesn't make a difference!