First, the word year is a reserved word in Access, so it should not be used as a field name (I used myYear as the field name in the example below). With that said, you will need some Visual Basic for Application (VBA) code to do what you want. The code will look something like this (air code, not tested)
Code:
IF DCount("*","yourtablename", "myYear=" & YourYearCombo & " AND NumID=" & controlname)=0 THEN
myincrementcontol=0
ELSE
myincrementcontrol=1+DMax("myincrement","yourtablename", "myYear=" & YourYearCombo & " AND numID=" & controlname)
END IF
I believe the code needs to be in the before insert event of the form, but i may be wrong. You will probably want some code prior to the above to check to make sure that a year was selected from the combo box and that the NumID was entered and is valid.