Problem Restoring Database On SQL Server Express '05, Vista

Hi All,
I have an issue that's stopping me restoring a database on SQL Server Express 2005 running under Windows Vista. I'm installing a system that I've used several times before on other systems and part of the installation procedure is to use the 'restore' function to install a database from a backup file. So I'm essentially restoring but to a db that doesn't yet exist in my SQL Server, this normally goes without a hitch. However on attempting this on a Vista system, I get the following error message -

System.Data.SqlClient.SqlError: The operating system returned the error '5(error not found)' while attempting 'RestoreContainer::ValidateTargetForCreation' on 'c:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLiCasework.mdf'. (Microsoft.SqlServer.Express.Smo)

where iCasework is the name I've given in the restore dialogue as the DB to restore to (which as I said, doesn't exist at the moment).

Things I've tried so far: -
I have a feeling this is down to a permissions problem, so I've tried the following already
1) Running Management Studio Express as Administrator - exact same error message
2) During the initial install, I set up the default admin user to use SQL Server authentication, having read threads about vaguely similar errors I used the Surface Area Config tool to add my windows login as an admin with admin rights and tried connecting to the Server with those credentials then running the restore - exact same error message

and

3) I also tried manually creating an empty db called iCasework and then restoring the external backup into that existing backup, which just gave a different error stating that the backup file didn't match the structure of the existing db. I could create the whole db structure manually but that would entirely negate the purpose of restoring from the backup and, more to the point, has never been necessary on any other system we've done this one.

Has anyone got any ideas that I could try next please?