It modifies system files like the Kernel and other stuff. Files that cannot be updated after the beta expires.

http://www.virtualbox.com/

That's what I use, you can find the Windows 8.1 Ent Evaluation here

http://technet.microsoft.com/en-us/evalcenter/hh699156.aspx

I think they give you a key by registering for 90 or 180 days to evaluate it and try it out for free. Make a virtual machine in Vitualbox.

I choose Virtualbox because it has a GNU/Linux and OSX version, unlike VirtualPC so you can run virtual machines on GNU/Linux and OSX systems.

Some people like VMWare, others QEMU, and some use what Microsoft has. But Microsoft has these evaluation ISOs for free for a limited time so if one runs out, just reformat and reinstall it in a virtual machine to test it out.