You don't need to have Windows Vista to run it. From a spec perspective, in general if your PC can run Office 2003, you should find Office 2007 runs fine on it as well.
I think most home users assume that they need to get Vista rather than just upgrading Office on their current Windows XP PC.
It is also worth pointing out that Vista is extremely successful - we are deploying at or ahead of the rate that Windows XP did after its launch. The delay in perception keeping up with reality has resulted in many wrong judgements about the ease of Vista with driver issues largely resolved by vendors. I was talking with John Curran, director of the Windows business in the UK, about this yesterday - I'll try and get an interview with him soon.
So now you are assured, why not download a free trial?
link to the official system requirements is here.