Server 2008

Hi!

I have one computer, that i do not use and i would like to use it as a windows server for domain and VPN access.
I have one desktop computer and one laptop computer and i have a couple of questions.

1. Can i have only one domain user for both computers (logged in at the same time), or do i have to create a seperate domain user for each computer?
2. At the moment i am testing the configuration. I can succesfully ping domain.net, server.domain.net, or computer1.domain.net, but I can not ping computer2.domain.net (on computer 2 it is reverse.. i can succesfuly ping domain, server and computer2, but not computer1). Why is that?
3. The computer i plan to use for server is an old P4 3.2Ghz. Will it effect speed on other computers, or will each computer be as fast as if not on domain?
4. What happens if server fails?

There are several benefits are there, some of them are live migration that moves virtual machines without downtime and direct access for connecting remote users without needing a virtual private network.