This is a useful thing for someone like Robin to have, but not what she's
looking for now. A slipstreamed CD is an original source CD--to use for
reloading XP from scratch, for example.
You can't use it to add the Service Pack to an existing machine, though.
Slipstreamed CD's should be made from the original media associated with a
given kind of machine--there are a number of different kinds of source
media: Retail, OEM, and Volume Licensed, as well as full and upgrade-only
versions.
So--it's quite a useful thing to have--if you originally bought XP Gold, and
your disk drive dies, you would normally have to install XP, then SP1, then
SP3. A slipstreamed CD would save the two SP installs. However, there are
complexities to it that the average user is probably not aware of.