To download the Microsoft SQL Server 2008 R2 August CTP edition go here
http://technet.microsoft.com/en-us/evalcenter/ee315247.aspx
Beware a SQL Server 2008 R2 Management Studio is not included in the Express edition. To get the Management Studio, you need to download the the Enterpise Edition.
The system requirements to install the software are as follows
System Requirements
- Supported Operating Systems: Windows 7; Windows Server 2003 Service Pack 2; Windows Server 2008; Windows Server 2008 R2; Windows Vista Service Pack 2; Windows XP Service Pack 3
- 32-bit systems Computer with Intel or compatible 1GHz or faster processor (2 GHz or faster is recommended.)
- 64-bit systems 1.4 GHz or faster processor Minimum of 512 MB of RAM (2 GB or more is recommended.)
- 2.2 GB of available hard disk space