Somehow the fonts for my system have been changed. Everything looks either bold or italic, and the font itself looks like it's a different font family altogether. I'm not sure how this happened, but it's quite annoying. The problem is system-wide, in that Windows itself, my browser and apps all display their menus/text with this different font.
This happened to me once before, but I can't remember how I fixed it. The things I have tried so far include: System Restore, SFC /SCANNOW, installing individual Arial fonts (I think Arial is the font that's been screwed up), using a registry fix which just changed the font to something a little less obnoxious, and changing the font substitutions via the registry manually. None of these has worked.
I'm using Widows Vista Ultimate SP1 x64.