First a link to nLite:
http://www.nliteos.com/
I have used this program several times recently and in the past to slipstream. This program makes your life so much easier, it's just almost unbelievable!
You have the ability to upgrade an old version of a Windows OS to the newest service pack, integrate hot fixes, pre-load those pesky drivers (especially sata and raid drivers for those of us that dislike floppy drives), and make an iso for easy burning. the program does much more too. There are some very good guides on the Web page.
Two very important points:
1. You must be using the version of the OS you plan to Slipstream.
2. You will be able to save your sessions so you can make changes and additions easily.
(repost of my thread on another forum)