How do I create a running “Count” in a MS Access Query?

I need to create a running Count in a query. Ie. The query first needs to count records by month, and then a running count of all of them.

The query would look something like:

Yr Mth [CountOfPSI #] [TotalOfPSI #]
2009 7 5 5
2009 8 3 8
2009 9 2 10