Actually, all I had to do was unplug the new hard drive that came with it (the S-ATA) and the hard drive from my old computer stopped freaking out.
Still some weirdness, my new video card (a lowly FX5200) seems to be installed twice, once as a 250 megabyte card and once as a .5 megabyte card. And Neverwinter Nights doesn't seem to like the onboard soundcard (Realtek something or other). And oddly, the hard drive only connects at a rate of 100 as opposed to 133 like in my old PC, so it's slower. But it works, and the Sims 2 looks much better. They actually now have fingers, instead of mitten like clumps. And cast reflections in mirrors.