part of the Slipstreaming process is that it now enters your windows' License / prodcuct key automaticly as the OS is installed...and other options can be set as well.
if you have an OEM install disk (not one from say "HP, Dell, Gateway, etc) -i.e. a disk you bought over the counter -- You can use that key in your newly crafted Slipstream disk. your new slipstream'd disk should be useable on almost any box.
**if you intend to use the CD & key from the manufacturer of your computer (HP, Dell, Gateway, etc) keep in mind that those disks are "tatooed" to be used only with that manufacturer's hardware. You may be able install the operating system on other machines, but the tatoo will not recognize the hardware and will prompt you to obtain a new license...giving you about a month to do so before it renders the OS unuseable.
**you can also create your new CD, complete with all svc. packs & most recent updates, to *not* enter the windows product key. This allows you to install the operating system as per the above. However, you will be prompted to enter a product key on one of the final reboots after the install. **that key must match the CD you used to make the slipstream'd disk.