It seems that somewhere between November and the new year, WHMCS modified their system requirements page, taking out Windows entirely:
* We validate WHMCS to run in Linux based environments running the Apache web server. Other environments such as Windows based configurations may experience compatibility issues and are not supported.
Not that this is a huge surprise, I mean Windows servers? YUCK! (ok, that's just a personal taste thing) It's got to be hard enough maintaining a Linux version of their software, so this doesn't surprise me terribly much. What does, however, kind of surprise me, is the fact that there was really no notification about this, it just kind of went unpublished as it were.