Database Connection - Post-install - Created A Fairly Substantial Program

Over the last couple months I've created a fairly substantial program for a friend in vb 2008 express to deal with the management of a fantasy football league, for its data storage it uses mainly an sql server 2008 express database for all the data, however there are a couple of text files for some unrelated fixed values.

I have spent the last week or so putting some vague finishing touches on it so that I can give it to said friend to test for me. However after working perfectly fine for me for the duration of the construction and also working fine on a couple of computers I installed and tested it on, upon giving it to my friend I find that (after installing) as soon as he opens up any form which relies on database data it spits out an error akin to the following:

Exception Text:

System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

[CODE]...

Loaded Assemblies:

Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4918 (NetFXspW7.050727-4900)
CodeBase: file:/Q:/Windows/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll

[CODE]...

I have used a combination of LINQ and SQL statements to query and and update the database, with LINQ for querying and SQL for the updating, due to this most of the database communication is done through a LINQ to SQL datacontext using the pre-generated connection string in my app.config file: <add name="FFL.My.MySettings.FFLv2ConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\FFLv2.mdf;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient" />

The only other time a connection string for the database is used is in an SQL execution function which uses the following: Dim connectionString As String = "Data Source=.\SQLEXPRESS;AttachDbFilename='" + Application.StartupPath + "\FFLv2.mdf';Integrated Security=True;User Instance=True"

The textfiles are also referred to using the 'Application.StartupPath + "folders/filename"' method

As for other details; I started this program on vb 2008 express but upon finding that the express version didn't have much in the manner of deployment facilities, downloaded a 90 day trial of visual studio 2008 pro to deploy the program as an msi install file. Operating system wise, I started the project on XP before converting over to Vista home premium + latest service pack (I think) which is what I've finished it on and am using currently, the 2 computers I successfully installed it on were both running XP SP3 and my friend runs a dualboot of Windows 7 and Vista (not sure what edition), he's tried it on both to no avail.

And for reference; to the above code/error, the database is called FFLv2.mdf and the project FFL.

The pre-requisites for my program (as highlighted by vb) are as follows:

.Net Framework 3.5 SP1
Windows installer 4.5
SQL Server 2008 express edition

I've tried changing the connection strings to specify a specific filepath, but that didn't help.