Sql Server Express not installing on Windows 2003 x64 SP2

I can't install Sql Server Express which I'm trying to do for a Symantec Backup Exec 2010 installation on a 2003 x64 server with Exchange 2007.  I've downloaded it so it's not on media, and I've downloaded it more than once to make sure it's not corrupt.  I get to the progress bar and then nothing happens and I end up having to end task (I've left for lunch and came back an hour later, nothing).  A colleague I was working on it with, spoke to Symantec support and they referred him to call Microsoft to fix the problem.  Sql installs it to the Add/Remove programs so I have to remove it, and that goes fine but I get the same result upon installing it again.  I'm getting the following error and I'm not having luck with the solutions I've found...

"Product: Microsoft SQL Server 2005 -- Error 2356. The installer has encountered an unexpected error. The error code is 2356. Could not locate cabinet in stream: Sql.cab."

Thanks much.