User does not have permission to perform this action

Question

Dear sirs,
i made a project (VB2010 express) connecting to SQL Server 2008 express on my PC running Windows 7 (W7): it works as expected.
Now,
i need  the same project run on a different PC (running Windows XP,  say WXP, with VB2010 express installed) of my private LAN, but still connecting to SQL server running on Windows7-PC. So i exported the project file to WXP PC.
After enabling TCP for SQL, opening ports and bla bla bla, i can "see" SQLEXPRESS instance (running on W7) from SqlServerManagementStudio running on WXP and i can modify, select, create tables and so on.
Then i changed connection string parameters in app.config to point to the dB:
<connectionStrings>
<add name="IlMioCapolavoro22.My.MySettings.MyCompanyConnectionString"
connectionString="Data Source=INGSERGIO\SQLEXPRESS;AttachDbFilename=z:\mycompany.mdf;Integrated Security=true;Connect Timeout=30;User Instance=True"
providerName="System.Data.SqlClient" />
</connectionStrings>
In detail, i changed .\SQLEXPRESS to INGSERGIO\SQLEXPRESS and AttachDbFilename =c:\temp\mycompany.mdf to  AttachDbFilename = Z:\mycompany.mdf (c:\temp of W7-PC is mounted as z:\ on WXP PC).
IF i do F5, i get the sql exception above.
Based on what i read in other forums,
it seems that the user connecting to SQL server (when the program starts) has not enough permission to access the database: frankly speaking, i don't know which user should i add permission for (my knowledge of SQL is poor).
The SQL Server Log file doesn't show anything (or what i am able to see).
.mdf file is in a directory where anyone has all privileges in reading-writing etc.
Can anyone help me?
Thanks
Sergio

Answer 1

Dear Sofiya,
thanks for reply.
TCP/IP, SQLServer.exe and SQLBrowser were already configured (Windows firewall was configured by adding "listening ports"
and not by changing scope to applications... i don't think this is a problem): in fact i can connect db remotely from SSMS (all privileges).
When you say:
"make sure the
user running your DB application in other computer has permissions to access the database"
you mean "windows user" ? if so, same windows user & password are accessing the 2 PCs, they both have admin-privileges on both PCs (maybe different operating system is a concern?). I didn't
create sql-user so far because my understanding is that in windows authentication mode it shouldn't be necessary or am i wrong? 
"..., we can attach the “.mdf” file the SQL Server Express and then use the same method (Connect to database with
VB) to connect the database." ---> VB doesn't let me do this, because if i try to test the connection
with the db file from VB, it errors saying: "the file is in a network path not supported for database files. A database with the
same name exists, or specified file cannot be opened, or it is located on UNC share." Somewhere i had read
that the VB EXPRESS doesn't support database remote connection, so it is necessary to do it by changing manually string connection.
Any ideas?
thank you.
Sergio