Depends which one is easiest for you, if you want to learn about linux a little more. You could setup ubuntu with samba so you can share files to other windows machines, you could also use a DNS service as recommended above so you can then run a web server off the machine too best way to do that is to setup apache.
If you want to get something quickly setup then Windows XP will be a very easy configure.
Personally I would go for ubuntu as I find it is a very stable OS for a server. I maybe lay off the server version though if your a linux noobie. The desktop version gives you more than enough options for a server.