Win XP Computer keeps dropping out of the network

Hi,

Have just replaced three computers, running XP Pro, they start up and log on
to the network. For no reason they drop out of the network the lights on
the Netgear switch say it is all plugged in. If I long on to the Netgear
dg834 firewall rooter and view attached devices the computer is not there.
This does not happen all the time. None of the other computers on the
network do this (also xp pro machines)