Look at what was told about your network. It leaves too much unsaid.
While the number issues are firewalls and accounts (I supply a prior discussion) some have odd networks that can't be supported. Always give a short description of the network. Here's a common setup:
Internet -> Modem -> Router -> PCs. For that, here's a discussion in which we cover the usual issues.
I am having troubles networking Vista and XP Home. I have a laptop and desktop. If anyone can help it would be greatly appreciated.