Unique entries

Hi just a quick question. (Hope I have got the correct message board)

I have a form which is used for data entry in Access 2000.

In the field serial_no I need all of the entries to be unique and not duplicated. Therefore if serial number 001 is already entered and is then attempted to be entered again how can I stop it from being entered and how do I get a msg box to appear.

I know you can set the field serial_no to be indexed to no duplicates but when you make a duplicate entry the msg box is a standard Microsoft Access error message can I change this? Is it possible to use some kind of validation in the form rather than the table?

Thanks for you help

Does the user need to enter this number? Why no just make the field an Autnumber field and remove the hassle.

But if you want to do your own validation you will need to use the onChange event of the text box and then run some SQL to see if its being used already