Here is the quick link from where SQL Server 2008 Express Edition can be downloaded.
You can download it with many additional details as described in following image. Click on above link to go to page and select desired version.
Additionally, please install SQL Server 2008 Express Service Pack 1.
You can read one of my previous article where I have covered SQL Server 2008 Express in detail SQL SERVER – SQL Server Express – A Complete Reference Guide.
Reference: Pinal Dave (http://blog.SQLAuthority.com)