This check list is not for everyone.  This checklist is intended to cover the majority of problem encountered while attempting to install a Service Pack.  This checklist is designed for Windows Vista, however most steps can apply to any operating system.

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Automatic Update is Microsoft's preferred method to get Windows Vista Service Pack 1
Windows Vista computers set to Automatically Update (AU) will automatically start receiving Service Pack 1.  Not all computers, just a limited number a day.  This will continue until all Windows Vista computers set to AU have SP-1 installed.
The Automatic Update process could take several weeks before SP-1 is installed on all computers.
To set for Automatic Update:
Open Windows Update
On the left side click "Change settings"

Select your preferred option.
Click OK and follow prompts.
Many people, myself included, feel Automatic Update is not the best.
You are better able to avoid internet issues and other possible issues if you have SP-1 on CD or at least in a file on the hard drive.
If you pay for bandwidth or you are limited on internet time, you may be better off using one of the options listed in Step 1 below.
Regardless which method you choose, look at the checklist and make sure your computer is ready.
Spyware, viruses, other malware, and other issues, whether known or unknown, are some of the major causes of issues relating Service Pack Installation.
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*****PRINT THIS CHECKLIST*****

Install Windows Vista Service Pack 1 this way:
1.  Do not use Express Installation.
Order Service Pack 1 on DVD or download the Standalone Installation. (be sure to select the correct language). 
Then burn it to a CD or DVD (about 435 MB) and keep for the future.  This media may be use on any and all your computers using Windows Vista in the downloaded language.  Regardless if you have one computer or many, download and burn to CD/DVD.  that way you will already have the Service Pack if it is necessary for you to rebuild this or another computer.  this download will work on all Windows Vista computers.  Microsoft would prefer owners of a single computerrecieve SP-1 through Windows Update, but having SP-1 on CD/DVD gives you more flexability.

2.  It is best to resolve any current issues BEFORE installing Service Packs.

3.  Data should be safe, back-up important data just in case if you do not already have a back-up plan.

4.  Check to make sure you have all updated drivers.  Also be sure any mission critical software is supported by SP2

5.  You should have at least 2gb free space for automatic Update from Windows Update.  For the stand alone installation, have a minimum of 8 GB free space.  If you are close to the free space requirements, you need to start looking for ways to free space or add another hard drive.

6.  Check with computer manufacturer to insure compatibility with Windows Vista SP-1, in the past there have been a few models with where the company has said wait while the manufacturer implemented a fix before installing a Service Pack.  This is especially important if you have something other than a Desktop such as a laptop, Tablet PC etc. 
 

7.  Run an updated virus scan. Or Scan for Viruses online. Sometimes it is a good idea to run more than one to ensure nothing is missed: Kaspersky Online Virus Scanner Panda ActiveScan Trend Micro House Call 8.  Run Ad-Aware (free version) Update if you already have it. And SpyBot Update if you already have it. You may want to use both of these tools to be sure all spyware is gone. Make sure all tools used in the above steps are updated within the last few days.  Do not assume recently downloaded/installed tools are up to date.

9.  Perform Disk Cleanup (all boxes checked)

10.  Ensure correct date, time and don't forget Time Zone.  This is a significant cause of errors relating to updates.

11.  Disconnect all peripherals (camera, scanner, printer etc)

When you are ready:
12.  Create a Restore Point in System Restore

13.  Physically disconnect the network connection

14.  Reboot the computer
Login as an Administrator, disable ALL unnecessary applications including antivirus.
Do NOT touch any keys during the process unless necessary.

15.  Install SP-1 by double clicking on it, follow the prompts.

16.  Reboot

17.  Connect the network connection

18.  Reconnect peripherals disconnected earlier