Shutdown issues are generally caused by a program and/or process that is
refusing to exit gracefully. The program and/or process can be from malware
or can be legitimate (such as an invasive antivirus like Norton or McAfee).
If you are using a Norton or McAfee product,  uninstall it and replace with
a better program such as NOD32, Kasperksy, or Avast (free). The Windows
Firewall is adequate for most people. With Vista, shutdown issues can also
be caused by old/poorly written drivers so make sure all drivers are
updated. See Step B. below for general driver directions.

A.The first step is always to make sure your computer is virus/malware free.

http://www.elephantboycomputers.com/page2.html#Removing_Malware

B. Drivers - The First Law of Driver Updates is "if it ain't broke, don't
fix it". Normally if everything is working you want to leave things as they
are. The exception is that heavy-duty gamers will usually want to update
their video and sound drivers to squeeze every last bit of performance out
of the hardware to get the fastest frame rates. If you're not one of those
people, you don't need to update your drivers if there are no problems you
are trying to solve.

Never get drivers from Windows Update. Get them from:

1. The device mftr.'s website; OR
2. The motherboard mftr.'s website if hardware is onboard; OR
3. The OEM's website for your specific machine if you have an OEM computer
(HP, Dell, Sony, etc.).

Read the installation instructions on the website where you get the drivers.

To find out what hardware is in your computer:

1. Read any documentation you got when you bought the computer.
2. If the computer is OEM, go to the OEM's website for your specific model
machine and look at the specs (you'll be there to get the drivers anyway)
3. Download, install and run a free system inventory program like Belarc
Advisor or System Information for Windows.

http://www.belarc.com/free_download.html - Belarc Advisor
http://www.gtopala.com/ - System Information for Windows

C. If the computer is virus/malware-free, drivers are current, and no Norton
or McAfee programs are installed, then do clean-boot troubleshooting to see
which program/process is the culprit:

How to perform a clean boot in Vista and XP -
http://support.microsoft.com/kb/331796

D. If you need more information, here is an excellent shutdown
troubleshooter:

http://www.aumha.org/win5/a/shtdwnxp.htm

Standard caveat: If troubleshooting the issue is too difficult - and there
is absolutely no shame in admitting this isn't your cup of tea - take the
machine to a computer repair shop. This will not be your local
BigComputerStore/GeekSquad type of place. Get recommendations from family,
friends, colleagues.

Malke