This is not a WWT specific but, but a limitation of Win XP. While WWT does run in XP, it not recomended. XP uses the older XPDM drivers that don't handle transparent Windows like Vista and Windows 7 do. Therer may be some newer display drivers that will
The same issue can occur in Windows 7 or Vista if Aero is turned off (XP compatibility Mode).
Jonathan Fay - Principal Research Software Design Engineer, Microsoft Research