Invalid Host Name

My network started in XP with a desktop and a laptop; everything worked!My laptop crashed and burned.I added a new Vista Laptop (ACER) and with difficulty got it to work on the network. The ACER malfunctioned (could not establish internet or network connections) and was sent out to factory for repair.Meanwhile, my desktop machine crashed and burned and was replaced with a new HP laptop which was successfully integrated into the existing (XP?) network.The ACER came back, can connect to the internet (wireless) but cannot see any other devices on the network.When I try to connect to the ACER from the HP, a message comes back "Invalid Host Name".I have MAC filter on and have scrupulously checked the IPs.