Actually I just did this yesterday, however, it was on SQL Server 2005 Standard edition. Not sure it applies to Express.

I opened SQL Server Management Studio, created a new query then entered and ran the following:

DECLARE @var1 nvarchar(50)

DECLARE @var2 nvarchar(50)

SET @var1 = convert(nvarchar(50),@@SERVERNAME)

SET @var2 = convert(nvarchar(50),SERVERPROPERTY('MachineName'))

EXEC sp_dropserver @var1

EXEC sp_addserver @var2, local

GO