If you are getting a 408 error from x-lite this means you are not receiving any response from the sip registration server that you are attempting to connect to with x-lite. There are a number of reasons for this but they could be caused by:
a) Your router
b) Your firewall
Try disabling any firewall software you have running on your desktop. You can also try connected directly to your cable/dsl modem and bypass your router to determine if the router is causing this problem. You could also try disabling the QOS setting which is under Options -> Advanced Menu -> Quality of Service.