limited connection

I have had a Belkin high speed wireless G Router connected to a Linksys ADSL2MUE modem and have had no trouble with the Host computer or my wi-fi enabled laptop.I've purchased a Dell computer for my Son,put it in the bedroom,and fitted a Belkin USB wireless adaptor.It was ok for the Internet for first 30mins and stopped working.it seems i have "limited or no connection" due to computer not being assigned an I.P.address.Please help with advice. Belkin support was poor and my ISP doesn't advise on 3rd party hardware! thanks,in anticipation, smc