Roaming Problems

Hi,
We have 4 access points in our offices. All with the same SSID but different
channels.
We have laptops using XP SP1 with all the patches.
When the AP's were first set up using WEP, users could move about and
reconnect to another access point successfully.
Now we have configured 802.1x authentication (PEAP) with Server 2003 and
IAS. Now a user can authenticate on one access point but most times cannot
roam to a different one. You can be sat right under the AP but the laptop
will still remain connected to the weak signal from the AP he first
connected to.
What should initiate the handover? Is there anyway to set a threshold to
force this?
Thanks,
RogerC