CodeBook.

1.Check the version of the MS-SQL server. 

Take reference to this:
http://support.microsoft.com/kb/321185/zh-tw In the official site, there are lots of edition to download.

I work on the 32-bit machine(win 7), so what confused me most is the difference between SQLEXPR32_x86_ENU.exe / SQLEXPR_x86_ENU.exe.

2. Then, find the appropriate package to download: Take reference to this article:http://blogs.msdn.com/b/sqlexpress/archive/2008/08/22/the-sql-express-2008-family-is-getting-bigger.aspx