I've worked with SQL Server 2005 Express until now. Now i have installed SQL Server 2008.
What changes do I have to do to my connectionstring? I cannot find any good example for a SQL Server 2008 connectionstring.

Here is my connectionstring for SQL Server 2005 Express:
Server=localhost\SQLEXPRESS;initial catalog=NHibernate;User Id=%USERNAME%;Password=%PASSWORD%

For SQL Server 2008 I have tried the following. Also I've changed the dialect to MsSql2008Dialect:
Data Source=localhost\MSSQLSERVER;Initial Catalog=NHibernate;User Id=%USERNAME%;Password=%PASSWORD%

What am I doing wrong?

With a default installation of SQL server, you don't get a named instance. So your string probably has to be:

Data Source=localhost;Initial Catalog=NHibernate;User Id=%USERNAME%;Password=%PASSWORD%

Thank you Wolfgang,

As everytime you're absolutely right.