Legally, you have to sign up with MS to become a System Builder in order to use an OEM license. This can be done online, on the MS website using a .Net Passport, which you simply register for. You can then download the OPK. You should then install the OEM XP using the OPK.
It is very probably technically possible to simply put the CD in the drive and install XP from there but, legally, you should follow the MS requirements.
Whether you do or not is up to you and your conscience. If it works, and passes validation OK, then it is very unlikely that anyone is going to investigate further. Never having gone down this route, I cannot say what the outcome will be.
SD