Don't have XP but in every version of Windows I've used (95, 98, and 2K) the method is similar though the details are slightly different:
open Windows Explorer > Tools > Folder Options > File Types, scroll down to the file type, in this case flac, click advanced towards the bottom of the window and change icon should be one of the options shown. Once there if the icon isn't shown, browse to the folder/program that has the icon you want to appear, choose it and "ok" your way out of all the windows and next time you click a flac file the flac logo should appear.
<edit> I type too slowly, just follow the moderators instructions above </edit>