A) Is it Vista 32 or 64 bit?
B ) Have you tried the three repair setting buttons at the bottom of the Settings / System Settings page (Rebuild Icon cache, Repair shell icons & Repair Icon Images)? This usually fixes 80% of my issues with missing and/or corrupt icons.
C) Have you tried other icon themes, to rule out that your current theme isn't broken/corrupt?
D) I don't really have a D, but it just seemed there needed to be one...