Peer 2 Peer with XP

Y need a lan card or onboard lan in each machine
a swtich of 12 or 16 port
cat 5 cable some rj45 connectors 2 each for the machine
in win xp go to network & dial up adaptor
go in properties of tcpip
give unique ip address for each machine
i.e. 192.168.0.1 to 192.168.0.10
subnet mask 255.255.255.0
in property of compter
computer name
give workgroup same for all

XP Pro on all machines.

Tcpip ethernet network.

Lee

The 'help and support center' in XP has a check list of what you need to buy (ether net card, wires, routers, et cetera), what procedures you need to go through and how to set up your network. In theory it's easy, ah, but there's where the fun begins. If there are firewalls on the computers other than the windows xp firewall, you need to configure them to allow the connections. You need to set up the router to pass things nicely in and out to the internet safely. You need to 'share' (tell the computer to allow connections to) the resources on the computers. You'll be cussing before this is done. I sure was. Bon chance, ami.

Chanmkr covered the list of steps quite well. I would point out
that in those last 2 steps, you may have to boot machines
several times in order to get the Workgroup name to "stick." I
found that some machines changed it back to the default name,
and I had to keep trying until they would finally remember the
Workgroup name I'd given all the machines.

If every machine doesn't have the same Workgroup name, they
won't be able to "see" each other on the network.

Let us know if you have any further questions. It's a much easier
process than it was before XP.

Although all the previous answers are correct, your friend should know that the maximum recomended p2p group is 8. you will likely see a number of problems crop up when all the pc's are on at the same time. for more info check out "peer to peer" on microsoft's knowledge base.
To be honest, with that many pc's, I would ante up for some low end server to handle all the file sharing. The cost is less relevant when you are spending endless hours trying to trouble shoot network crashes and data collisions and corruption that will occur in that crowded environment.
good luck

Setup a similar peer to peer network for a friend, rooms on first and second stories of a house, I used a 4 port router/switch (1st flr) and a 9 port (2nd floor). I ran incoming Internet to the Uplink on the router and then three machines into the Router/Switch, ran the forth out port into the 9 port uplink on the second floor and hooked the remaining machines into that switch (had to do some drilling). Used the Network wizard on the XP machines to setup shared folder sharing. The router/switch had a printer port so put the drivers into all machines for a common printer. (NIC cards in all machines)

Well the above answers will work unless you have one computer that everything is trying to access for something like Files or whatever. If you are going to attempt to use one of the units a a file server you are at the absolute limit of concurrent connections allowed in XP Pro if you are using XP Home the limit is 5 concurrent connections but Home can not be added to a domain either and would benefit from a server application and a domain setup. But if this is nothing more than a glorified Games network it should work without a problem although you could get by without a router it is not a good idea but just make sure that there is no single computer feeding the entire network for Internet Connection.

If there will be no Internet Connection a simple 10 -12 port hub will do the trick and while wireless might look cost effective at first when you take into account what you have to do to secure it a wired setup is far cheaper.