Hi,
Try to download Microsoft SQL Server 2008 Express Edition Service Pack 1 with the following link and install it:
http://www.microsoft.com/downloads/details.aspx?FamilyID=01af61e6-2f63-4291-bcad-fd500f6027ff&displaylang=en
If it still fails, please provide us the exact error messages you get and the error messaged logged in the setup log files which are located in the %Program files%\Microsoft sql server\100\Setup bootstrap\Log\ folder.
Additionally, if you encounter any error when installing Visual Web Developer 2008 Express, we recommend you go to the following forum that dedicates Visual Web Developer 2008 Express:
http://forums.asp.net/1125.aspx
If there are any more questions, please let me know.
Thanks.
***Xiao Min Tan***Microsoft Online Community***