Take a look at this article - http://msdn.microsoft.com/en-us/library/ms345408(v=sql.90).aspx. You can start SQL Server at special configuration (single user and only master database is up) and then issue alter database for tempdb. In order to start SQL Server with master database only, you can run this command:
NET START MSSQLSERVER /f /T3608
After SQL Server starts you can run the alter database statement, and then restart the server.
Adi
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