Does any one know an easy way to convert a series of weekly data into their
respective monthly averages.

I have a large range of data in weekly format:
1/2/99 3.54
8/2/99 5.41
15/2/99 2.10

through to...
25/2/05 4.10

How can i get that data into a table which looks like:
1999 2000 ....... 2005
Jan 5.12 1.24 ...... 5.28
Feb 2.14 3.54 ....... 1.79
Mar etc etc
.....
Dec 2.45 4.8 ........ #NA

The monthly average table needs to look up the respective months in the
weekly data and average them into the corresponding cell (ie Jan-1999 equals
5.12 from the 4 or so weeks of data).

I am thinking vlookup function, but am unsure how to add average when
looking up a series of dates in a lookup function.
I already have a similar table which looks up the week number of the date
and puts the corresponding weekly data into the table against its week number.

The weekly data may also have returned #na for graphing purposes, is there
anyway i can accommodate this in the formula.

I would appreciate some help.