Did you buy a Retail or OEM/System Builders copy of Windows?
If you bought the retail, then you should have got two discs and keys that go with 32/64bit installations, which you can use multiple times (max of three I think).
If you bought the OEM copy, then you get one disc and one key, and its either 64 or 32bit depending on which you bought. The key can only be used on the motherboiard it is first installed too.