tweakUI will not
change your buttons, and is in fact for customising certain things in XP.
you want to be looking at custom themes: Theme - How to Apply Custom Themes
for example, here are my buttons:
however, be aware that themes don't just change buttons, but a lot of other UI elements, like the taskbar, start-menu, OK/apply/cancel buttons etc etc
also be prepared to spend a lot of time browsing through deviantart for the theme of your choice!