LaurelLee done it for you except....switch the True and False statements with each other and place the code into the OnClick event of your ComboBox:
If cboYourBox="No" Then 'If NO is selected...
'If the textbox is actually empty... Then disable it
If IsNull(Me.txtYourtextboxname) then Me.txtYourtextboxname.Enabled = False
Else 'Otherwise YES must be selected therefore make sure TextBox is Enabled.
Me.txtYourtextboxname.Enabled = True