What is the OS? If it's XP (or higher) then chances are you won't. Don't quote me on this, but the last full OEM OS Disk I ever got with a PC was Win 98. Usually you either get a recovery disk (or the option to make one) or a restore portion on your hard drive.
May be wrong, but I think that giving away a full OS CD and code with a new PC was stopped mainly because of MS changing licensing rules as a result of open abuse by this policy.