Check out the DMax() function in the help. You can
certainly use it in an expression in an update query and I
_think_ (but can't remember for sure) that you could use it
in the default value property of the appropriate control on
your form.
Something like =DMax("[SID]", "table or query name")+1
should automatically increment the SID field by 1 each time
a new record is added and the user doesn't have to do it
(and probably shouldn't).