In the Choose SQL Server computer, if you select “Use the existing database..” radio button and press Next, a Database Login dialog box is displayed, requesting a username and password. Typically the Server dropdown will be something other than SERVERNAMESQLEXPRESS.
This can happen if 3rd party programs, such as ACT, install their own database instance.
The best way to work around this is to run the SQL Server Express setup manually. You can download this from http://msdn2.microsoft.com/en-us/express/bb410792.aspx.
Within the Setup program, choose to allow both SQL and Windows authentication. SQL Server Express will then be configured with an instance name of SQLEXPRESS.
You may then re-run Softalk Share Server setup and select SERVERNAMESQLEXPRESS from the Server dropdown in the Choose SQL Server Computer page.