Finally found an alternative to Project Diva. It's called Stepmania, and it's been ported to PC, and it does essentially the same thing as Project Diva. Any song can be used for it though (I've just put on a lot of vocaloid for now). Anyway, here are the required links:

Stepmania Download (choose StepMania-3.9.exe)

Stepmania Patch Download (required)

Patch Installation Instructions

Vocaloid Song Downloads

Hard to get the hang of, but it works great.

It's also incredibly easy to add/make your own songs for it, check this tutorial