If you mean buy it then you probably can't anymore. If you haven't already got a copy, you'll be able to obtain a copy of XP from a local computer shop.

1) Put the XP cd in.
2) Restart, taking care to ensure that the CD drive is set higher than the HDD in the boot order.
3) Follow the install instructions on the screen.
4) You're done!

As NDM says, you will need to purchase a legitimate XP installation disc, complete with the product key. (I would steer clear of eBay copies as they may be pirated versions).

After August of this year you will be unable to purchase new copies of this disc as Microsoft will not be supporting or producing it anymore.

Now that the first Service Pack is available for Vista, you are best off sticking with it.