Microsoft provides a free version of its data management system SQL Server Express 2012 for Windows 7, Windows Server 2008 R2, Windows Server 2008 Service Pack 2 and Windows Vista Service Pack 2

Microsoft® SQL Server® 2012 Express is a powerful and reliable free data management system that delivers a rich and reliable data store for lightweight Web Sites and desktop applications. It can be used to store the arcplan user management database "arcplanDB".

The setup for arcplan version 8 includes arcplan Engage needing the SQL Server Advanced Services contained in the Express with Advanced Services Setup of SQL Server Express 2012: "SQLEXPRADV_x64_ENU.exe" or "SQLEXPRADV_x86_ENU.exe"

http://www.microsoft.com/en-us/download/details.aspx?id=29062

Update Jan 2016: The most recent version of SQL Server Express with Advanced Services 2014 can be found here:

https://www.microsoft.com/en-us/download/details.aspx?id=42299