Vista - Low disk space free problem

I'm usually running low (~5GB free of 60GB) on free disk space, but I've
noticed an 'odd' behaviour with Vista that has just hit as a 'critical'
problem.

When I copy files off of my computer over a network to my file server my
free disk space drops. Today I was copying a 8GB VirtualPC disk image so I
could compress it and copy it back and I've run out of disk space, which has
caused all sorts of problems with other applications that periodically write
to the disk, including other file copy / move opperations off of the
computer over the network. Furthering the problem, when the copy completed
the disk space used by the copy was not released. As you might assume, I'm a
bit hesitant to reboot in case it still isn't cleared and the OS crashes
when it fails that initial read. I'm quite sure this isn't a pagefile
problem as I don't use a dynamic pagefile.

Where is Vista caching this so I can delete it? How can I stop it from doing
this in the future?

I didn't think I'd find Windows Explorer cyclicly crashing and recovering
because there's no free disk space left my Vista showstopper, but here it
is, and I need to know how to clean up after the OS if it's not going to do
so itself.