Manually install SQL Express when it fails to install automatically. See our article on installing SQL Express
Manually download and install the SQL Server 2005 Express Edition SP2
Copy the link below, paste it into your web browser and click Go.
Scroll down to the heading Files in this Download. Select SQLEXPR.EXE (55.4MB) and click Download.
When the popup opens, click Save. Do not click Run. When the Save As popup opens, ensure that Desktop appears in the Save in field and click Save.
When the download is complete, return to your desktop and click the SQLServer2... icon to install as illustrated below and select the default settings as you go through the prompts.