Power Supply Unit failing to provide the wattageGPU or RAM failing/overheating
But it can easily be the motherboard too especially the PCI-e slot (or AGP if older)
1. run a hardware diagnostic software. could help identify the problem.Link
2. trace the problem by process of elimination. tedious but effective.
[Edited by saurabhfzd, 4/13/2011 9:46:02 PM]
though it would be weird,since most computers,specially the new ones,were high def and upgraded well...
also maybe your anti-virus or firewall is blocking WoW...