VMWare, for example, was around before hardware virtualization was common on desktop computers. So was QEmu as another example.
What happens here is that VMWare/QEmu would emulate the virtual CPU and hardware, instead of virtualizing it. This is much slower.
Neither VMware nor QEmu currently support running on a system without hardware virtualization (I could be wrong about VMware but I doubt it).
However, older versions of these programs (probably very old at this point) may work, if you can find them. I remember being unable to run VMWare Workstation on a 533Mhz AMD K6-2 Compaq due to lack of proper SSE support, but was able to on a built 1.3Ghz AMD Duron system. (I will tell you VMWare with 256MB of RAM is not pretty). I forget the specific versions involved but it did work. Barely.
You may have problems running Windows Vista or 7 under them, though.
To actually answer your question, if your processor doesn't support hardware virtualization, you can't enable it. If you have a crappy BIOS that disables it even though the system is capable of it, I have heard of BIOS mods that allow it to be enabled. However, your CPU physically does not have the feature so you are out of luck.