I have 3 computers 2 running win ME and mine running XP, they are all
connected to an SMC ASDL router, two by wire and one by wireless. I
want to be able to turn off access to the other two computers at
specific times, ie after 2300hrs, and at certain times at the weekend,
as my two daughters are constantly online. Is there specific software
to do this?

your router is capable of setting certain time blocks for an IP
address / IP address range , this does require that either you give
your daughters machines a fixed IP address or that you give your own
machine a fixed IP address and exclude the rest of the range