Depending on your system you may be prompted to press a key (usually F10 or ESC for HP systems) to enter the BIOS right after you power on the system. In the BIOS you should find the VT-x (or "Intel Virtualization Technology") setting in "CPU-Options" or "Security". Make sure you enable it and then save the BIOS settings. Once saved you need to power-cycle the system (i.e. power-off/power-on) in order for the changes to take effect.
André