Win7 shutdown stalls

I shut my computers down at night, but in the morning I find the "Windows is shutting down" window still displayed. This is recently happening a lot, and is also happening on two of my other computers including a WinXP laptop. Where do I start to look for reasons?
When this occurs, there is nothing for it but the 5-second salute, as the machine is completely stuck, but there is no "unexpected shutdown" message when I reboot. Occasionally, it will get stuck on the "Downloading 1 of 2 updates", and killing that DOES result in a bad-shutdown message.

Event viewer doesn't show me anything I can use...

Doesn't sound like the source, Windude99: the Win7 machine runs Norton (NIS) and the laptop is running AVG...

Ztrucker: I usually shut all running apps and tasks down before I quit a session, so the Task Manager is almost always empty on shutdown. On occasion, I get a message saying Windows is waiting for Sticky Notes to close, but it always resolves in less than a minute. I treat this as a quirk, because Sticky Notes is always on, sometimes it appears as a Task, sometimes not. That being said, I will pay attention to TM in the future.

When I told the guy at Geek Squad about the hanging-shutdown problem, he immediately declared it to be a corrupted Windows file, and the only thing for it is the F8>Repair Windows route. Any thoughts on that? Sounds draconian to me.

After two weeks of normal shutdowns and no hangs, I believe the offending program was the Post-it widget/gadget (that came with Windows). Since disabling it, no further problems. Thank you Ztrucker. Oddly, the XP machine doesn't have that widget, but just stopped having a problem - perhaps out of sympathy or loneliness?

After combing through similar threads as this one, I discovered my experience with delayed start ups and hanging shutdowns had to do with a failing external drive. If you having this issue I would first unplug all external drives then boot PC... If there is no delay in start up or shutdown One of your drives is the culprit. Plug them in one at a time... Power up each time to discover which one is failing. Here's a thread I found after the fact related to this issue. external hard drive causing system to freeze on boot and shutdown