I found this bug through Geany, but they suggest I should post this to you.
Basically, Geany's font rendering on Windows XP is sub-optimal compared to SciTE on the same machine, or the same Geany version on Windows 7. Here's a screenshot on XP with Geany on the Left and SciTE on the Right. http://i.imgur.com/x6X3q7u.gif
Geany Packages GTK+ 2.16 with their editor, and I believe Scintilla 3.2.something for the current stable release.
Scintilla has a call for setting font rendering quality but its not hooked up to anything on GTK+
http://www.scintilla.org/ScintillaDoc.html#SCI_SETFONTQUALITY
There may be a Pango or Cairo call to use for this but I couldn't find anything in the Pango documentation.
Sometimes buffering can get in the way of quality rendering.
http://www.scintilla.org/ScintillaDoc.html#SCI_SETBUFFEREDDRAW
May want to investigate http://www.cairographics.org/manual/cairo-cairo-font-options-t.html#cairo-font-options-set-antialias .
Windows XP is no longer supported by Microsoft so closing as won't-fix.