Connection string fails - permission denied

[Guest]

The following error appears when I try to "view in browser" (in Visual Web
Developer) a data-driven Web page:

SELECT permission denied on object 'Stocks', database 'EzMarket', schema
'dbo'.

I use Windows authentication and the Web.config file has the following
connection string:

<connectionStrings>
<add name="EzMarketConnectionString" connectionString="Data
Source=3Z4L0YQHJ3LWW5M\SQLEXPRESS;Initial Catalog=EzMarket;Integrated
Security=True;"
providerName="System.Data.SqlClient" />
</connectionStrings>

I also tried to create a new user with full permissions on database
(EzMarket) and table (Stocks), with no result.

I'm very confused of that. I'd be very grateful for any help.

Many thanks