I need some security help

Hi,

I was asked to do some project. Our company is not doing good right now. Some people are leaving and we are kind of concern about the security of our network.
We have a windows2000 server. I’m just wondering what’s the best way to protect our server. We changed the password for our administrator account already but we’re still concern for the people who is leaving and knew some of users password. We are afraid that they might sign on as a different user and do something to our network user folders and network configuration.
We are also using Citrix. I have no idea how the security on this work. Please help. Thanks.