Networking and Energy Saver

I have a wireless network in my home with four computers.
They are all linked to a router so it is an adhoc
network. Two of the machines are running XP and 2 are
running Windows 98SE. The machines with Windows 98SE are
always accessible and run on the network just fine.
However, whenever the two XP machines go into standby the
files and resources on those machines are not accessible.
I tried disabling hibernation with no luck. The systems
are set to turn off monitors and hard disks in 15 minutes
and go into standby in 20 minutes.

Does anybody know what the most energy efficient means of
keep the network running full time? Is standy the
problem? It seems to defeat the purpose if every time I
need to use files on a machine I have to go wake that
machine up first.

Thanks,

Kyle