Network runs fine with open security and not with security

I have a wireless network of 4 computers. 2 x XP desktops 1 x XP laptop and a
new Vista desktop. These are networked via a Belkin router. I have absolutely
no problems with the network when it is open, however as soon as I enable
security settings I encounter problems. The printer (which is conected to the
Vista machine, although visible, is not able to be accessed by the XP
machines, both Vista and XP machines although showing that they are able to
access files on each other do not. As soon as I disable the security settings
and become an Open network straight away all three XP machines can print
through the Vista machine and I can access and move files around the network
no probs. I have had the Vista machine for 9 days now and have spent about 6
hours each day attempting to sort one problem after another out, I have had
it back to the technicians at the shop twice for them to resolve simple
problems for me and I am getting very frustrated by this whole Vista thing.
Help me please someone, I know a little but thats all.