<p>Is there a way to lock a row in the SQL 2005-2008 database without starting a transaction, so other processes cannot update the row until it is unlocked?</p>

Everything you execute in the server happens in a transaction, either implicit or explicit.
You can not simply lock a row with no transaction (make the row read only). You can make the database read only, but not just one row.
Explain your purpose and it might be a better solution. Isolation levels and lock hints and
row versioning
.