1st step is to see if your CPU supports virtualization or not.

2 ways to do that: lookup the specs on your CPU or use an app that tells you, for instance Everest Ultimate 5.4 does this (haven't checked, but I think Speccy by Piriform, the CCleaner folks, does this too)

One of my machines supports it (Core2 Duo E6400) and the other doesn't (Core2 Duo E4700). So no running VirtualBox on my HTPC.

Secondly, enable it in BIOS.