Paul,
As Al states, XP Pro is a limit of ten. If you are using one database such as Access and one program, peer to peer is fine. If you are using an office which needs a file server, application server, possible web server (although never a good idea to support a web on your main server and possible Exchange Server (numerous advantages), then you need a server. A server in and of itself has many advantages with way greater throughput. Also, a server sits there doing its job for weeks, sometimes, month at a time with barely a hiccup for the reasons above and not needing to be tweaked that often. It also tends to run a server-based OS which offers things like Active Director and a domain controler. Running a domain on a network, although harder at first, again, has numerous advantages.
But, like the volkswagen vs the Maserative, some times all these aren't necessary. If one were first deciding, sitting down and weighing the advantages of each would be helpful. While a volkswagen is nice, I think given the option of the two, if someone offered me a $4,000 server with a $500 OS, a $1,000 back up program, $1,000 or more Exchange (all of which come with SBS), possible Sharepoint (company web), Active Directory, and extremely more redundancy than a peer to peer, it would be hard not to go for the latter.
Just my three cents.
