You might also look into www.autopatcher.com.
I updated Windows
2000 pro with autopatcher and have had no problems.
The single downloaded file when launched brings up a very nicely laid
out menu of all the updates it will apply, you can check/uncheck what
you want and apply only those fixes you want and ignore the rest.
Just read the info provided at their site if at all interested.
Otherwise, you need to download all the updates, fixes etc from
Windows Download Ctr and save them to your PC and install them one
at a time. If you can figure out how to use "qchain" you can create a
script that will chain all your saved files and only reboot once when
done but I cant figure out how to do it so that is why I went with autopatcher.
Look under "Search For A Download", and the "Product/Technology"
search for everything related to your version of XP. Click "Go" and a
very long list of updates, ALL things related to XP will pop up.
I like to sort by Date and Newest First after I do search and every
update, fix etc is listed from Microsoft by the most recent release.
Windows update is quicker if you have broadband just simply install
everything at once and dont bother saving for a later install.