choose best network automatically

Hi
I have 2 wireless routers at home, one on each floor.
When I move with my laptop from one floor to the other, the reception the other router is bad but still exist, therefore the laptop does not connect to the new network.
this causes bad internet connection, although a better network exist.

how can I make windows 7 (or with a 3rd party software) check the available networks periodically (let's say every 1-2 minutes) and if another better (with a higher RSSI or quality) network exist in range switch to it?
thanks