What is "apache"? From your description it seems to be a DSL router.
Assuming both home and office networks are using DHCP and your dad is putting the XP laptop into hibernate rather than powering it down, does he run ipconfig /release when he leaves his office and ipconfig /renew when he connects at home? If not, he should.
http://compnetworking.about.com/cs/w...wipaddrwxp.htm
"To bring a computer back onto the network after moving it to a different location, or experiencing an unexpected outage, first release, then renew the IP address. Computers on DHCP networks often (but not always) re-establish network connectivity automatically."