Various VM applications will let you install an operating system in a virtual environment on your Windows desktop or laptop. But not all are free and not all support the latest Windows 8 beta.
A product such as VMWare Workstation 8 supports Windows 8 but costs $200. Microsoft's own Windows Virtual PC is free but won't accept Windows 8 as a guest.
Instead, you can use Oracle's VirtualBox or VMWare's VMWare Player 4.0. Both are free and both support Windows 8.
To get started, you'll first want to download the Windows 8 beta, aka Consumer Preview, as an ISO file.
You can download Windows 8 from here. After selecting the Visit site button, click the button to Get it Now and then choose the option to "download Windows 8 Consumer Preview in ISO format instead." You can download the ISO file as either a 32-bit or 64-bit version. Both VirtualBox and VMWare Player support 64-bit Windows as a guest. Note the Windows 8 serial number as well.