Depends on the EULA. Not all copies of XP are the same. Some allow for only one installation, others allow for two simultaneous installations or more. Some, especially student versions and some OEMs, don't allow you to install Xp more than 2 or 3 times before the disk goes dead.
You should be able to install it on one desktop and one laptop, as long as you are not using them at the same time. At least that's how it works here in the UK.