ASP.NET Machine Account is created when the Microsoft .Net Framework is installed onto a Windows XP machine. By default the Account is called "ASP.NET". Please make sure that you have Microsoft .Net Framework installed. In higher versions of Windows the account is called NETWORK SERVICE.
You could also create a new user account in the SQL Server and set its username and password in your connection string. This user should be dbo over the database that you are going to use with Sitefinity.
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the Telerik team
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