Error 25P02 - transaction aborted - Potential causes?

I've started to get the following error:

25P02 - current transaction is aborted, commands ignored

This error occurs on a 'select' statement done within a transaction, after multiple selects and updates have already been made. The application is written in VB.Net, and uses npgsql for data access. (I'm running postgres 8.2).

Does anyone have any idea what the cause of the error could be? Is there any chance that its because Postgres needs more space for doing rollbacks? (If so, how is this space increased).

Or, is there any change I can make to the way I do the transactions that will help? Currently, I'm using the following:

Dim conn As Npgsql.NpgsqlConnection
Dim Trans As Npgsql.NpgsqlTransaction
conn = New Npgsql.NpgsqlConnection("my connection string...")
conn.Open()
Trans = conn.BeginTransaction

Are there any parameters to the transaction that I can use that would prevent this error?