Could someone please show me a formula to average weekly data into monthly.

I have a spreadhseet:
Date Data
2/1/99 1.15
9/1/99 2.42
16/1/99 1.24
through to.....
25/2/05 3.54

How can i calculate the average of every month between 1999 and 2005?
In other words, i need something which looks up all the weeks in a
particular month (ie Jan 2001) and averages them out into a table:

1999 2000 ......... 2005
Jan 2.45 6.42
Feb 3.64 etc etc.

I was thinking vlookup, but don't know how to average in a vlookup.

I would appreciate some help.