The SQL Server 2008 RC0 download is available to the general public here.
For goodness' sake, don't load this on a real machine. Go get Virtual PC or Virtual Server - they're both free. You can get the Trial Editions of Windows 2008 Server here:
Windows Server 2008 Standard (x86 and x64)
Windows Server 2008 Enterprise(x86 and x64)
Windows Server 2008 Datacenter(x86 and x64)
Windows Web Server 2008(x86 and x64)
You can also get a configured virtual hard drive here - already installed and ready to run.
Regarding virtual machines: invest a day now if you are not familiar with this technology. It will save you weeks and months in the future, I promise.
After you get your virtual envirnoment set up and running, you can install anything you like. This protects your laptop or desktop from "bad things" happening with a CTP or RC installation. You will thank me later.
Using Virtual PC and Virtual Server, you have to install the x86 versions of the OS and SQL Server 2008 RC0. For me that's no big deal - I'm not loading it to stress it (not yet anyway), I'm loading it to learn more about it. You can too.