One of my clients had a server with a somewhat limited C: drive. The server would occasionally fill up with log files that were hundreds of MB in size under the W3SVC folder with the same entry over and over... some kind of web request from a particular Vista machine to my server hitting WebDAV. This one plagued me for a while, but I finally sat down and traced where the error was being generated from.
According to a few internet sources, apparently there is some kind of problem with Vista and WebDAV, or more importantly Web Folders. I won't go into the details as they do not matter really... what's important is that Vista machines attempting to access a server running IIS and WebDAV may generate endless amounts of these WebDAV-Miniredir entries in your IIS logs. Eliminating whatever application is making these web requests is the key to preventing the log bloat.
In my case, the machine had Quickbooks 2009 installed, and along with it some kind of unused quickbooks web connector. The connector was set to run on startup (even though it wasn't doing anything), and thus whenever the vista machine was on it was generating a few lines of logs per second. Removing the web connector from the startup folder fixed the problem.