If you are looking at Debian as a operating system, I can tell you that our experience using it with Moodle 1.5, & 1.6, etc. was very good. On older equipment (Dell 2650, 2GB ram, slower Mirrored Raid drives)
At that time we were using older PHP 4.x and MySQL 4.xx, and older apache 1.x, but our Debian server ran w/o any errors that required rebooting for about 3.5 years straight.
Yes I did say years... you can get really spoiled with that kind of stability.
Debian Linux puts a huge premium on stability and it shows. the downside for us was more complex updates and patches that only a very few of our staff knew how to do. We have been using Ubuntu Server lately and it has worked well too . It doe have a much larger, more active development community, but it is NOT as stable as Debian was.
We tested windows 2003 server, PHP, Apache 2.x in about 10 different setups and were not able to get away from the well known "memory leak" issues. If you decide to use windows for Moodle it appears that there are some groups of components PHP 5.x, Apache 2.x.x, that do work. I would strongly urge you to copy a "stable" setup exactly (as described below by one poseter) if you choose windows.
We found out the hard way that many combinations of Window 2003, PHP 5.x and Apache 2.x.x are most certainly NOT stable. YMMV