This is a quote regarding OEM Windows XP from the microsoft forum from a Microsoft employee:
"With the version of XP that you have, the systembuilder/OEM version, you are allowed to install it on the original computer as many times as you want. You are NOT allowed to install it on any computer other than the one it was first installed on. Changing the motherboard (except when replacing a defective with an equivalent) is considered changing the computer."