Sounds suspiciously like the old PC may have been set up for Internet Connection Sharing which would not be so unusual if for example it was the first computer to be on broadband using a USB modem.
Reason this would cause problems is that it would be offering DHCP and DNS services to the network (acting like a second router with no internet connection).
prompt $P - Invalid drive specification - Abort, Retry, Fail? $G
prlzx on iDNET: ADSL2+ / 21CN at ~4Mbps / 700kbps with IP4/6