32 bit software on a 64 bit machine is fine, the other way around is not. (I doubt you need 5.5, though. If you're more comfortable with 64 bits, you could just grab a stable release of whatever reasonably recent version they have.)
I would however take Xero's advice and install a complete server environment like XAMPP. Or install a complete virtual Linux server. (Bitnami or turnkey would be good places to start, they have pre-built LAMP stacks available as ready-to-run virtual machines or as ISO's which can be installed into your favourite VM.) However, the latter may not be as "accountant friendly". And it will be scarier.
On the other hand, it will be a "contained" installation which will not interfere with Windows. And you could easily move the whole virtual machine to another host if you wanted to. Not to mention how much fun it will be. And how much you will learn. If you don't believe me, just go with XAMPP.