I really doubt that system fonts would be named like the files calibre is complaining about. Usually on windows you have two sets of system fonts, both from microsoft, both named as simple ttf files in the c:\windows\fonts directory. But then, I'm no windows guru
@itimpi: Actually the link not found message comes from the SVG rendering library, when it loads an SVG icon for the interface and the icon has a bug in it (an xlink element whose href is empty or points to something that doesn't exist).