Logged in users see the personal links at the top - My talk, My preferences, My watchlist (id="p-personal") - vertically rather than horizontally. Some links are also non-clickable, because they appear behind the search box.
I tested this with a new account on he.wikipedia, ar.wikipedia, fa.wikipedia and mzn.wikipedia, and it happened everywhere, so it's probably not caused by a local customization.
This happens only in Vector; Monobook is OK. And this happens only in Internet Explorer 9; IE8, Firefox, Opera and Chrome are OK.
OS: Windows Vista