While it is true that Ubuntu 7.10 is quite old. If you are in particular of using the same Ubuntu flavor instead of upgrading it,.. let's see..
From Aspire 4736z specs, Atheros ar8131 is your lan controller. Do confirm if this by typing
#lspci | less
And look out for Atheros or Ehternet controller or Network controller like that.
You are sure that Atheros ar8131 is your LAN controller. Do visit this http://ubuntuforums.org/showthread.php?t=1255082
You dont know to figure out from the lspci output. Please post your lspci output here.
You may want to upgrade you Ubuntu