Disk Usage in Vista RC1

>I installed Vista RC1 last week on an old PC and, for the most part, the
>experience has been great. I'm getting the full "Ultimate" version,
>including Aero. And the performance is actually better than XP.
>
> However, I do have one puzzling issue that I'm hoping someone can help me
> out with.
>
> So that I wouldn't wreck the installation of XP on my existing IDE hard
> drive, I popped in an old 13.6 GB drive for the installation. I realize
> it's very small, but it's mated to a 160 GB SATA drive. So, I install all
> productivity and gaming apps. on that drive.
>
> Even with that, I notice that the amount of available space on the primary
> drive flutuates wildly, without seemingly any reason. It does this whether
> or not I've cleaned out files. Typically, it's about 1 GB of space that's
> in play.
>
> I'm stumped, and haven't seen this behavior on any prior version of
> Windows. Does anyone know what might be causing the variation in disk
> availability?