modem contacting the DHCP server problem

hey all.. today i had a callout to go and fix a computer..

when i got there the internet worked fine on it (expect msn messenger - which was the reason i was called out) but he wanted me to check for virus' etc..

so the first thing i do is use ad-aware and find a massive 821.. so i got rid of this.. ran anti-virus - found a whole bunch.. but i noticed half way thru of ad-aware process.. the internet had gone off completely..

so the computer is cleaned up.. viri free etc.. restart.. internet is still off.. everything is fine with the cable modem..

so i look around for a bit.. the configuration is fine.. i use the "repair" option when in the status of the network card.. it says "connect my ISP Administrator because renewing the IP Address failed"

there is no value for the default gateway

ipconfig/renew gives this message : An error occurred while renewing interface Local Area Connection: unable to contact your DHCP server. Request has timed out.

so i formatted and re-installed xp..

same problem

neone know how to fix this problem?

thanks -des