SQL for Deleting all duplicate entries

Hi,

I want to create a DELETE statement which deletes duplicates
in a table.

That is, I want to remove all rows - but one - having three
columns with the same data (more columns exist and there the
data varies).
For example:
column1
column2
column3
column4
column5

column2 = 'test', column3 = 'hey' and column4 IS NULL for
several rows in the table. I want to keep just one of those
rows.

Is this possible? I can't figure it out, so any help MUCH
appreciated!

/regards, Håkan Jacobsson - System developer at Relevant
Traffic, Sweden