I have done it using vCenter Converter and it works perfectly well from inside
the computer,
provided that the disk on which the VM is written is not included
in the conversion. The copy was of good enough quality that Windows stayed
activated. You may need to register an account in order to download the product.
However, my last usage of it was on Windows Server 2003 32-bit using an
MBR partition table, while I suppose that you are using Windows 8.1 64-bit
with GPT and UEFI boot. This shouldn't present a problem, since
VMWare KB 1016992 affirms this capability since version 5.1.x,
while current version is 6.1.1.
But theory aside, the only real test is to do the conversion and see whether
it works for you on your computer, and furthermore,
whether Windows 8.1 stays activated after its conversion to VM.
The conversion process might take hours to complete, and my advice is
to include in it only the system drive.
As further note: Windows 8.1 contains Hyper-V, so you could use
Microsoft Virtual Machine Converter to do the P2V conversion.
I have never used this tool, so cannot discuss it from personal experience.