P2P?! Are you a network admin or a PC tech?
An XP file server in a small business environment will inevitably be used as a workstation too. This is the worst possible scenario. Users break computers, period! Part of the point of a file server is to keep filthy user hands off of the data store. Users will open email attachements and browse to sites that will cripple your XP "server". The cost of SBS is only $300 (see viosoftware.com part #T72-00661-cdo) versus $100 for XP. Spend the money and keep users and their filthy habits away from the mission critical data. Imagine the downtime when you have to reload that corrupted system because you let a user use your server. P2P has no place in a business environment. Fault tolerance and server level security are not expendable, cost saving options. I won't put a bad solution in place for anyone regardless of their budget. Have the courage to say that you'll give them an appropriate server solution so that their business can succeed or that they can take their business elswhere.
While I'm ranting, don't you dare put a server in with a celeron or P4 processor. While they may not need the processing power of a Xeon, what you get when you sell them a Xeon processor is a server class motherboard. Workstation motherboards are for workstations. Cutting corners here will cause heartache.
Your small business customers don't want a server any more than they want to pay taxes and insurance. What they do want is for their business to run smoothly and that means minimizing downtime. You're just asking for trouble and downtime that is far more costly than doing it right from the beginning. You're not doing yourelf or your customer any favors to cut corners. Give them what they need not what they want, they will always want to spend less. Downtime is far costlier than the difference between an XP "server" and a real fault tolerant server.
ALL businesses need a server!
