Groovesalad,
What this code is telling you is that MYSQL is attempting to alter a table known as THREAD by adding a column called TPOSTID to it. Notice the error: "Duplicate column name 'tpostid'"?
This means that it already exists.
Here's what you do:
Go into myphpadmin or sql server admin of choice, drop that table.
Run the install for 2.1 THEN do setpostid.php if and only if your news doesn't show up.
Hope that helps.