Next time, run the setup on a fast machine, then port the Additions-installed VHD over. Duh.
There's a reason the screenshot is one of setup! It's been running since about 8pm tonight (it's 10:30pm now, and it's been 13 minutes to completion for about 30 minutes).
The Additions will eventually boost the speed immensely, but as I'm running a virtualized P3-700 with 300MB RAM (the host has a massive 512MB total), and without the Additions (because we haven't finished Setup yet), well, you just have to wait.
... I can wait.