could be conflicting drivers. try running your single wow and do this:

go to options/video/resolution and make sure 'windowed mode' is CHECKED
go to options/sound and make sure 'sound in background' is UNCHECKED
go to options/sound and make sure 'use hardware' is UNCHECKED

apply the changes... exit wow (this forces the settings to write to disk)

now try to restart more then one instance and see of that helps.

if not, it could be memory