У вас я так понимаю SQL7, а цитата из BOL для SQL2000, но все же объясняет ситуацию

"In a DELETE, INSERT, or UPDATE trigger, SQL Server does not allow text, ntext, or image column references in the inserted and deleted tables if the compatibility level is equal to 70 .The text, ntext, and image values in the inserted and deleted tables cannot be accessed. To retrieve the new value in either an INSERT or UPDATE trigger, join the inserted table with the original update table.

When the compatibility level is 65 or lower, null values are returned for inserted or deleted text, ntext, or image columns that allow null values; zero-length strings are returned if the columns are not nullable.

If the compatibility level is 80 or higher, SQL Server allows the update of text, ntext, or image columns through the INSTEAD OF trigger on tables or views.
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