i'll start by assuming your playing windowed mode. sometimes vista 64 and WoW have issues generating the fancy little vista preview screen when u mouseover the program on the start bar.

to fix this right click on computer(equivalent of My Computer for XP), select properties, choose the advanced tab, click the settings button under performance, and select the bubble for adjust for best performance.

this will make vista look almost exactly like windows XP so if u enjoy vista for the visuals you will be disappointed, but this solved all the issues me and my friend were having with windowed WoW.