It's one of those secret things you find out by accident, and kinda makes you wonder how it slipped you by, (pardon the pun). Just type the phrase "slipstream" into Google and you'll wonder how you never got to hear about it.
This is a damn handy preparation tool that is fully automated within windows. It just requires you to issue a couple of commands in DOS.

So... what is it?
It's the process by which you integrate a service pack into your operating system and then create a bootable CD for it. The result is you get an up to date copy of Windows without the need to spend hours installing hotfixes, patches and packs in order to get it bang up to date.
I slipstreamed with the help of these two articles, but there are a few more listed below that you may also find useful.

Webtree
PC Stats
One Computer Guy
blogs.linux.ie
Help with windows

Windows2000 and WindowsXP have a very similar process.