If you want to see if the eMachine's wifi card is working, open the 'Device Manager' under 'Start > Control Panel > Performance and Maintenance > System > click the 'Hardware' tab and select 'Device Manager'.' A window will pop-up. Expand the item 'Network adapters.' If the wifi device is installed correctly, it should show there. If there is a 'red X' on the icon, it means the device is disabled. Re-enable the device by right-clicking on it and selecting 'Enable.'
In the case that the wifi device in enabled but not working, you'll want to reinstall the drivers. Just download and install the appropriate driver according to whichever showed in the device manager (Broadcom or Atheros).
Atheros : ftp://ftp.emachines.com/notebook/E725/XP/08.%20WLAN_Atheros_V184.108.40.206_XP_WHQL.zip
Broadcom : ftp://ftp.emachines.com/notebook/E725/XP/08.%20WLAN_Broadcom_V220.127.116.11_WHQL.zip