It looks as if you have the OEM version. Did your disk come in a rather plain box with square corners on the disc's storage case?
I only know of OEM discs being shipped as singles rather than the set of two with the retail and upgrade versions.
Now the question is, would a product key of an OEM 32 bit version work with an x64 OEM version? For example; a Home Premium 32 bit OEM to Home Premium x64.
My question is ... is it legal ?