Is your optical drive still working? If not, these are fairly easy and straightforward to replace.
You will need to do a clean install if you want to go from 32-bit to 64-bit (or vice-versa). If you want to create a USB key to make installation easier, you can do so from within either a 32-bit or 64-bit environment. Simply follow this Tutorial USB Windows 7 Installation Key Drive - Create
I have created USB keys for all my listed OSes (see my specs), and have used 8GB keys for each, which leaves enough space to include Service Packs and other useful drivers
and programs on the same stick. This makes installation a breeze, as everything for that installation is all on one key which can be updated as and when the need arises.