The preferred method of installing service packs

The usual method of installing a service pack in either Windows 2000 or Windows XP is to go to Windows Updates. Windows updates will detect that the service pack is required, and then download an installer that will scan your pc to determine which portions of the service apply to your system, and then download just there portions and install them. There is a better method however, known as the network install because it can be done either locally from a cd or your hard drive, or over a local network. In the network install you download the entire service pack and then install it. This is the preferred method of installing service packs for several reasons:

The service pack can be downloaded once and stored on your local network or burned to CD and used to upgrade any PC, avoiding your having to download it over the Internet each time you wish to upgrade a computer.
This method can be used to upgrade computers that are not connected to the Internet, by installing from a CD.
It is not necessary to be connected to the network during the installation, avoiding exposure to viruses and worms on computers that are not yet fully set up. This is a very important for Windows 2000, which comes without an internal firewall.
The upgrade can be performed in safe mode, where there will be a minimum of background tasks interfering with the installation, and a minimum of opened files.

Once you have downloaded the service pack, the method of performing the network install of a service pack is the same for all service packs for Windows 2000 on up through Windows XP and Windows 3000. The method of downloading the service pack is somewhat different for Windows 2000 and Windows XP, let's cover that first.

Windows 2000

Go to http://www.microsoft.com/. On the left side of the page under Product Resources click Downloads.
On the left side of the new page under Product Families click Windows.
Find the drop-down box labeled Show downloads for: and select Windows 2000 from the drop-down list.
Find Windows 2000 Service Pack 4 Network Install for IT Professionals and select it. (Note: do not choose the express install. This is the same version of the service pack you would get at Windows Updates.)
Select Download, and save the file W2KSP4_EN.EXE to your hard drive. If you will be installing it on the computer use to download the service pack, save it to the root of your C: drive. If you are planning to burn it to cd: for use on another computer, save it to your Desktop. This is a 129.2 MB file.
If you wish, after the download completes burn it to a CD-ROM.

Windows XP

Go to http://www.microsoft.com/. On the left side of the page under Product Resources click Downloads.
On the left side of the new page under Product Families click Windows.
Find the drop-down box labeled Show downloads for: and select Windows XP from the drop-down list.
Find Windows XP Service Pack 2 for IT Professionals and Developers and select it. (Note: do not choose the express install. This is the same version of the service pack you would get at Windows Updates.)
Select Download, and save the file WindowsXP-KB835935-SP2-ENU.exe to your hard drive. If you will be installing it on the computer use to download the service pack, save it to the root of your C: drive. If you are planning to burn it to cd for use on another computer, save it to your Desktop. This is a 266.0 MB file, by the way!
If you wish, after the download completes, burn it to a CD-ROM.

Installing the service pack

Boot the computer on which you are planning to install the service pack into safe mode. See How to Reboot to Safe Mode for instructions on doing this.
Logon using the built-in Administrator account.
Double-click on the service pack network install file, either in the root of your hard drive, or on CD.
After the service pack installation completed, you will be asked to reboot. Do so. Your computer will now reboot to normal mode. The reboot will take longer than normal, as there are some additional steps to the service pack installation that will now be performed.

Note that you can always update directly to the latest service pack without having to install intervening service packs. Higher service packs always contain all of the necessary material from lower service packs.

Note also that repair installs remove service packs down to the level of the CD from which the repair was performed, after which the service pack must be reinstalled. You never know when you might have to perform a repair install, so this is a reason to keep service packs handy on CD.

And finally, for Windows 2000 users may I once again point out that on a freshly installed system with no firewall, if you are not behind a hardware firewall such as the one in a router, the chances of your getting service pack 4 downloaded without picking up a Blaster variant are less than 100%, so hang onto the service pack for later use if you need it.