Cannot install the 'AdventureWorks 2008 OLTP' database.


I have downloaded and executed "AdventureWorks2008_SR4.exe" and I am unable to install the 'AdventureWorks 2008 OLTP' database.
During the installation, I have confirmed that the option 'AdventureWorks OLTP 2008' is ticked, and the comment next to it says 'Ready to install'. My installation instance name is SQLSERVER2008, and I've used 'SQL Server Configuration Manager' to 'Enable FILESTREAM for Transact-SQL' and 'Enable FILESTREAM for file I/O streaming access' with the Windows share name as SQLSERVER2008.
I followed the steps at http://msftdbprodsamples.codeplex.com/wikipage?title=Database%20Installer%20Help using the 'SQL Server Management Studio (SSMS) approach'.
I get this when i run the script:

Started - 2010-04-05 15:36:30.297

*** Dropping Database

*** Enabling FileStream
Configuration option 'filestream access level' changed from 2 to 2. Run the RECONFIGURE statement to install.

*** Creating Database
Msg 5133, Level 16, State 1, Line 2
Directory lookup for the file "C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\AdventureWorks2008_Data.mdf" failed with the operating system error 3(failed to retrieve text for this error. Reason: 1815).
Msg 1802, Level 16, State 1, Line 2
CREATE DATABASE failed. Some file names listed could not be created. Check related errors.
** An error was encountered during execution of batch. Exiting.

My database folders in the script are thus:
:setvar SqlSamplesDatabasePath "C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATA\"
:setvar SqlSamplesSourceDataPath "C:\Program Files\Microsoft SQL Server\100\Tools\Samples\"
Im using SQL 2008 with Windows XP SP3. Im a 'Computer Administrator' when I login.
Can anyone help as I'm completely at a loss.