Hi
To add to Mike and Jim, you need patience as well. I have updated two pc's with sp2 and on an average the install alone took about 1.5 hours/pc for me. So it will help that you plan it when you have time and more over after the install you need to carefully check all of your existing software to evaluate if they work as expected or not.
And for the first install read up all you can about xp sp2.
Cheers