You would only have the option to install 32 & 64 bit if you purchased windows 7 as a retail product.
Any operating system bundled with a system will, under most situations always be an OEM product. Which apart from costing you 150$ less then it's retail alternative, is locked to the system it is pre-installed on via the eula agreement you had to agree to when starting your new system.
If you just want to test the 64bit on a different partition I would agree with Molly and install the Trial Enterprise edition of the software. When the trial runs out you can rearm the expiry date 3 times to squeeze another 90 days out of the install. Instructions can be found here