From the link below, you will note that the cpu pentium d 830 does not support VT.
Before getting any new CPU, make sure to check if the option to enable VT is found in the bios. It may under security settings.
Then check from the list of compatible CPUs which one supports VT.
All the best,
Try replacing the CPU... I've recently replaced my 840 CPU with a 965 and VT (which was not available) is now available in the BIOS.