<p>I'm wondering if there are any simple ways to get a list of all fixed-width (monospaced) fonts installed on a user's system in C#?</p>

<p>I'm using .net 3.5 so have access to the WPF System.Windows.Media namespace and LINQ to get font information, but I'm not sure what I'm looking for.</p>

<p>I want to be able to provide a filtered list of monospaced fonts and/or pick out monospaced fonts from a larger list of fonts (as seen in the VS options dialog).</p>

AFAIK you can't do it using BCL libraries only. You have to use WinAPI interop.
You need to analyze 2 lowest bits of
LOGFONT
.lfPitchAndFamily member. There is a constant FIXED_PITCH (means that font is fixed-width) that can be used as a bit mask for lfPitchAndFamily.
Here is a useful article:
Enumerating Fonts
Enumerating fonts can be a little
  confusing, and unless you want to
  enumerate all fonts on your system,
  can be a little more difficult than
  MSDN suggests. This article will
  explain exactly the steps you need to
  use to find every fixed-width font on
  your system, and also enumerate every
  possible size for each individual
  font.