IF you Go to Control Panel - System and look at the section where it has the activation status if it is an OEM product it will have OEM in the Product ID line just below the Window is activated line !
Even if it is OEM you can use it to change to 64 bit -- The product key will work for either 32 or 64 bit installs interchangeably BUT you will need to get an install disk for the 64 bit version (the retail version comes with both disks where OEM version only includes the 1 disk --- If you have an OEM version you can get the install media iso from Digital River (which is Microsofts approved digital download source -
Link to article with various links for the images
You will need the image that matches the version you have ( ie. home premium, Ultimate, professional, etc. - but the 32 or 64 bit versions are interchangeable) - So download the correct version for the license you have and create a bootable iso image to use for installation. --- NOTE : be sure to back up anything you need before reinstalling as switching from 32 bit to 64 bit requires a clean install - meaning you have to format the HDD and thus will lose any info on that partition !!