Dear rico:
When you create a number field in Access, the field is given the default
value of "0". Ensure that the default value setting for the field is blank,
and then any new records will have a null value in that field. You change
this value in the table design view. If you need help to do this, post back!
Please note that this will only affect *new* records, it will not modify the
existing values in the table. For that you would use an update query.
HTH
Fred Boer