Auto fill a access form field

In Access 2000, I want to look in a form field and apply a value to another field in the same form based on what is entered in the first field. For example:
If field A=1
Then field B=Matt

The value of field B is not a lookup from another table.It is only dependent on what the user enters in Field A. What is the easiest way to accomplish this?

T.I.A.