You need to identify the active transactions! SQL Server will not truncate the log past the active transactions and who knows what other havoc they could be causing due to locks that are being held.

As you've probably found out, DBCC OPENTRAN() will provided information about active transactions. Here's a sample from my server:

Transaction information for database 'pubs'.

Oldest active transaction:
    SPID (server process ID) : 54
    UID (user ID) : 1
    Name          : user_transaction
    LSN           : (6:139:1)
    Start time    : Jun 13 2001  8:28:21:710AM

You can use the SPID and sp_who2 to retrieve enough information to help track down what's happening.

EXECUTE sp_who 54

Specifically look at the HOSTNAME column which will contain the name of the computer that opened the transaction and ProgramName which might contain the name of the program on the machine.

You will not be able to properly correct your situation until those open transactions are handled.

-Mike