excel 2003

have long lists of data entry (column one days, column two week average),
need to be able to drag down along column (B) rather than retype specific
cells to average, problem comes in when dragging it will average 2:3
procedding average 1:2

A B
1 average 1:2
2 average 3:4
3
4

sounds simple but I cant figure it out, would greatly appreciate any help.