I think it comes down to a matter of niche. The important question is: Why do people want to install OpenACS on Windows anyway? In my case, I had a small site, no budget, and a machine that needed to do double-duty as a Windows workstation. Others want it for development work or a simple sales demo on a Windows laptop. (With VMWare a strong competitor for these uses.) And I imagine there is some market for rapidly deployed intranet or small non-profit sites. I could be wrong, but I can't believe anybody wants to run large e-commerce OpenACS sites on Windows.
Some sort of sponsored mod_aolserver or FastCGI solution could definitely benefit the entire community. Cleaning up, packaging, and documenting AOLserver on Win32 would just be much less work. Maybe use the saved time to create some new modules or improve some old ones? At any rate, I'm sure any work you can contribute will be warmly received. In addition to the good suggestions this thread is generating, you could try out the existing options (native AOLserver, mod_aolserver, ISAPI) over the course of a week or so and anything you learn would be useful.
I'm not the best person to answer Michael's question about AOLserver features in v4 vs. v3. According to an OpenNSD bboard posting by Scott Goodwin:
"AOLserver 4.x is more aggressive at optimizing conn performance, has a new comm API that is tremendously simplified, has virtual server capability put back in, plus more."
I'd guess new sites will increasingly use AOLserver 4, but I don't think you need to worry about AOLserver 3 becoming unusable in the near-to-medium future.