For now I'm going to leave Windows ports as something for other people to do downstream if they want to. It should be pretty easy, and I don't want to do it myself because I don't have access to a Windows machine or understand anything about the OS.
Currently, ensure-dir and date use system to run Unix commands. It would be much more portable if they used mzscheme operations instead. This has caused me trouble not only on Windows, but different versions of Linux. And I think make-temporary-file could replace /dev/urandom.
I changed date to get the date from Mzscheme, but it's not so easy to change ensure-dir. Mz's make-directory doesn't create intermediate directories like mkdir -p, and I don't want to get into trying to understand pathnames.
If anyone wants to take on being a distributor for a Windows port (or for different variants of Linux, for that matter), I do have a patch for date here: http://catdancer.github.com/date.html
Another option is the Anarki stable branch (http://github.com/nex3/arc/commits/stable/), which has most of fixes necessary to make most of Arc work portably on Windows and other OSes. (And, being a bug-fix branch, the amount of other random material is limited.)
I agree with the option, but I also think these posts will fall off the deep end in about 2 months. By then new members may only discover the option after they've discovered the problem. So we're not really saving new members wasted efforts unless the install page guides people.
pg: Even if you'd rather not think about supporting Windows portability yourself, providing a link to the Anarki stable branch would help new Windows users. Finding this stuff on the forum after it's fallen off the top couple of pages is not very easy.
I have most of them already fixed myself, with the exception of pipe-from, but an average person (aka me) with Windows OS is going to spend time both trying to get Arc working and sifting through the forum which isnt easily searchable (I had to sift through over 300 posts, with 80% of the content being way over my head, to determine pipe-from wasn't expected to be working for a specific reason.... had no clue what "dev/urandom" even was).
If I might suggest: if your not going to fix these that you start a known issues document to save people from the headaches and tail chasing. Even putting something on your install page noting that Windows isn't supported would be appreciated by newcomers.
[Edit] As an idea maybe we could have one posting thread for OS related issues that your install page has a link to ? ....