There was a shell tweaking utility (not TweakUI) I stumbled across a while back that let you change the text of the start button. Poking around, I realized that it was actually twiddling some bytes in the Explorer.exe file. I'd guess that the image for the start button (the little Windows flag) is probably in there, too. If so, its probably not something the casual user will want to mess with. But if you're determined, I'd start in the executable.