Problem attaching stored procedure and SQL Server trigger

All goes well when I execute it in SQL Analyzer. So, I tried to attach this stored procedure to the trigger. When

a certain column in one table changes, I want the trigger to call this stored procedure. It seems easy, but when I tried it, the SQL Server instance blocked. I got a message saying there was a permissions problem of using the method sp_OACreate. I've modified the authorization, but it still won't work.

When I execute the stored procedure and trigger separately, there's no problem. Is there any difference between these two ways, or did I lose something in my test?