Not sure if this will help.
Double click on the Wireless Icon on the RHS of Task Bar. In the Wireless Network Connection Status box that appears, click on Properties.
In the Wireless Network Connection Properties box, click on the Wireless Network tab.
In the Preferred networks area click (highlight) your network in the box. remove any unwanted networks using the Remove button.
After highlighting your network, click on Properties then in the new window, click on the Connection Tab.
In the Connection tab window make sure that 'Connect when this network is in range' is ticked. If this does not work you need to confirm that the signal strength is not the problem.
Also I would recommend Windows 7 as the wireless network management has been improved.
Hope this helps.