My recommended approach:

1.  Don't buy a computer with pre-installed OS or anything else.  Rather, build your own.  It's easy and there are lots of resources out there to help guide you.

2.  When you build your own and qualify for OEM version of OS, you get the disk (cheap) and can reinstall whenever you want (or need to due to bad drive etc).  And you only install the software and services you want.

3.  Avoid Vista until it is stable and has adequate drivers... probably after at least the first service pack.

For recent build, I bought XP Pro with free (except $11 S&H) upgrade coupon for Vista.  Left space for a future dual boot partition for Vista once it is stable.  Only reason I'm installing Vista at all is that it is required for some upcoming games my kids will want to run.  But they will not get to do that until Vista is stable... maybe a year from now?

Oh, and my new custom duo core build takes 22 seconds to do a firefly render which it took my Dell workstation over 3 minutes to render.  I'm sure part of the reason I get these speeds is the lack of Musicmatch, Real, and other unnecessary (for me) pre-installed programs and services.

Poser 7 is also very flaky and flawed (e.g., the "point at" bug), but it's fast at rendering.  That 22 second render in P7 takes twice as long in P6.  And if I build the scene in P6, the point at bug doesn't pop up when the scene is then loaded into P7 for rendering.

My bottom line: Build your own machine using XP Pro SP2.  Avoid Vista, at least for now.   Build scenes in P6 but render the frames in P7.