I just did a restore default and this is what it put it back to as a default font:
MS Shell Dlg 2 [8pt]
Is that correct?
I found all of the Liberation fonts in a folder at C:\Program Files\Calibre2\resources\fonts\liberation but the do not show up as a choice in the list when you select "Change Font" under "look and feel"
I'm confused now (not that that is really unusual!)