In the ON CHANGE event of your combo-box, insert code similar to:

Select Case CombBoxValue
  Case This, That, SomethingElse
     Me.Checkbox = 0
  Case Else
     Me.Checkbox = -1
End Select

Fiddle with it a bit, but that's essentially what you need. Be sure to
single-step through the code to make sure the CASE works for all values.

If the Row Source of the combo box is a query, you can add a field (Y/N -
boolean) to the table that would indicate if the service needs a script.

If you want to know if a service needs a script, query the table. There is
no chance of someone accidently changing the state of the checkbox.

If you still want to keep the checkbox, it can set it using the column
property.

If the combo box row source fields were

lngServicesID (PK) (long)
txtServiceDesc  (text)
ynScriptRequired (boolean)

and the control names are RequireScript (check box) and cboService (combo box)

then the checkbox could be set/unset using

Sub cboService_Afterupdate()
 ' the column property is zero based. The first column is numbered 0
  Me.RequireScript = Me.cboService.Column(2)
End Sub

Just another way to remove the cat from its skin

HTH