I replaced a Core 2 Duo E6300 with a Core 2 Quad Q9550 E0 in a Windows 7 machine. XP Mode has stopped working - gives 'cannot start host process...no hardware assisted virtualization' error. Ran securable it reports a big red No for hardware virtualization. Gigabit MB, no BIOS settings related to VT and no MB changes since it worked. Not overclocking. Every spec I can find says Q9550 has hardware support for virtualization. Is this a defective CPU? Intel Core 2 Quad Q9550 Stepping A Revision E0.
More aboutintel q9550 reports hardware virtualization
Have you recently updated your bios? If not, give that a go and see if it comes up then. If not, it very well may be a faulty chip and hopefully, you can return it.
Always reformat the OS after a CPU change; not doing so can cause problems like this. BIOS update may also be needed for proper support of later quads.