When you purchase a MS OS all you really purchase is the serial number. So load up your current copy of XP, enter the serial number given with your new disk and see if it activates. If it does, you probably have a legal copy and if it doesn't it's return to E-Bay time and your new copy has been unopened.
Assuming of course that the serial number is on the outside of the packaging.
And then if you want to make double sure, go My/Computer/Help/Is this copy of Windows Legal?
And Microsoft, being the wonderful people they are, will probably let you know in no uncertain terms whether your copy's a hooky one or not.