Configuring the Build Server's system-directory

I just had a very unpleasant surprise when I noticed that my Windows
server-side(!) profile's size had increased to way above 1 GB and most of this
was caused by the build server's artefacts that were copied into the
system-direcory.

So, how can I configure the system-directory to point to somewhere else? The
profile-size issue is probably not such a big problem on a production system,
but even there it feels odd to have such large amounts of data buried in a
home-directory...

Thanks,
Sascha