Your computer ships with a 64-bit EFI implementation. Such an EFI can boot only 64-bit OSes in EFI mode. Thus, to boot a 32-bit version of Windows, you must boot it in BIOS mode. This requires converting the partition table (not just a single partition; the entire partition table) from GUID Partition Table (GPT) form to Master Boot Record (MBR) form, since Microsoft has decided that it won't support booting in BIOS mode from GPT disks. Most disk partitioning tools can make this conversion in a destructive way (wiping out everything that's already on the disk), and a few can do so non-destructively. My own GPT fdisk (
gdisk) is one of the latter. Note that MBR partitioning goes by a number of other names, such as "MS-DOS partitions" or "BIOS partitions." Thus, you might need to look for the right name, depending on what tool you use.
Note that even if you convert the partition table non-destructively, your existing Windows 8 will probably stop booting. (I haven't tried such a conversion myself, so I'm not 100% positive of that.) Essentially, you must boot all your Windows OSes in one mode (BIOS vs. EFI), at least if they're installed on one disk. If you want to dual-boot, this could be a problem, since given your stated goal, you may have to re-install Windows 8 in BIOS mode. This in turn will require access to a retail version of Windows 8, since the OEM restore tools will probably only restore in EFI mode.
This might be a bit easier if you're willing to run the 64-bit version of Windows 7. You could then install it in EFI mode rather than in BIOS mode, which would not require any partition table conversions and would enable dual-booting, if that's something you want to do.