There’s a bug in the current build of SQL Server Express which prevents it from being installed on a Windows 7 machine (the pre-installation checks will fail on a “restart required” link. Restarting, obviously, doesn’t fix this.
Go to Start > Cmd > RegEdit and navigate to the following key:
In the right-side pane, double-click on the key “PendingFileRenameOperations” and clear any values in there. Exit RegEdit.
Then, just re-run the SQL Express intaller and all should be fine.