Are you using any A-V, anti-virus? Can you restart into Safe Boot, Shift at the chime; give it much longer than a usual boot. Can you shutdown properly from a Safe Boot?
If it shuts down from a safe boot, that will generally mean some third party item is preventing a proper shutdown. Safe Boot disables those so that points the finger at one of them.
It you installed Snow over Leopard or Tiger or migrated everything over from a previous OS, there might be something incompatible. Or you may have installed some third party program since doing the Snow install.
This may help to find the culprit: fellow contributor etresoft has written a small program that may help identify what's interfering. Unzip it and run the app, then post the results here. You would run this booted normally.
Also, run this command in Terminal in Utilities and post the output. Just copy/paste it in and hit return.
kextstat | grep -v apple
It is possible, although unlikely, that the Safe Boot itself fixed the issue.
Message was edited by: WZZZ