Try Windows 8 Without Losing Windows 7

You shouldn't replace your current OS with one that's not yet ready to be sold commercially. If you want to install Windows 8 in a way that leaves Windows 7 unharmed then I recommend installing it in a virtual machine (VM). Virtual machine software imitates an actual, physical computer, onto which you can install another operating system. This is the easiest and safest way to try out an operating system.
I recommend Oracle VM Box for creating and running your Windows 8 VM. It's free, open-source, powerful, and it already supports Windows 8. (It's main free competitor, VMWare Player, doesn't support Windows 8)
You'll also need to download the .iso file version of the Windows 8 preview. This Page contains both 32- and 64-bit editions. You'll also find the product key here; you'll need that during installation.

Once you've installed VirtualBox, follow these directions:-

• Load VirtualBox, the click the New icon in the upper-left corner.
• This starts a wizard. Click Next.
• On the "VM Name and OS Type" page, enter a name that includes the phrase windows 8. Otherwise, "Windows 8" won't show up as a Version option.
• Stick to defaults for the result of the wizard, even when asked if you want to "Create new hard disk." Don't worry, it's just a virtual disk.

 

• When the Wizard finishes and closes, double-click the new virtual machine.
• Read the message about the Auto capture keyboard, then click OK.
• The First Run Wizard will start. When you're asked for Installation Media, click the folder icon on the right and select your downloaded .iso file.
• The installation will begin after you click Start.
• When asked for a type of installation, select Custom. Don't worry about overwriting files. The only hard drive the installation sees is the empty, virtual one.

>> When you're done, you'll be able to run Windows 8 inside Windows 7.