Here's my story...I've got Vista home premium. With Aero enabled, guild wars works smashingly in both windowed and full-screen modes. The minute I turn off aero, things get a little screwy. Full-screen is still ok, but windowed produces the following effect whenever I right click to open a context menu, or hover over a start menu item to bring up a bubble, etc.
That's fine, I've dealt with it and avoided it. Except today comes along, and suddenly the effect is permanent

I've rebooted, I've reinstalled graphics drivers. The only things different today is I installed that fancy vista "performance" pack that MS released
So basically, I have to go back to using the molasses-inducing aero, or play full screen...neither of which are friendly options to me /sigh