Windows Calendar problem

Hi there,
This is a bit of a silly problem but it's driving me nuts.
When I'm in Windows Vista calendar I might have a reacuring appointment that
says maybe 'Take kids to school'.
I'm a bit daft but I do like to mark stuff as done, so say I 'took the kids
to school' I would like to go into that appointment and mark it as done (by
maybe typing #), BUT, I can't get it to change just that appointment, it
changes all of them (so it would look like I took the kids to school everday
even in the future lol.
I click 'change just this occurance' but it still changes all of them.

Any ideas?