I have a couple of clients who just have a box running XP (or 2000) as their server.
If you have 5 client PCs, use GoDaddy for $5/month for web & email, you really may not need any apps running on the server. So why not just get a $400 XP box (incl. UPS), and use Mozy ($5/mo for 30 Gb) for online backup, and you're good to go.
I agree it's good to have file and print services all served from 1 PC, but for a lot of very small businesses that's all they need.
Yes, if you know them and want to save a few bucks, go ahead and use Linux of FreeBSD, or whatever's your favorite. They all so file and print serving just fine.
Actually, in my experience, even XP (Pro) does a perfect job at this: I've never seen a crash in an XP box just running file & print services, and 1 GHz / 256 Mb is plenty, as long as you don't measure performance while you're logged in.
If you've never tried Mozy, do yourself a favor: by default, it wakes up every couple of hours and does an incremental backup to their servers in Berkeley. Throttling (my idea!), encryption, and all that stuff included. I'm very happy with it. FREE with 2 Gb for home use. Get an extra 250 Mb if you use this link:
https://mozy.com/ref/VKXPV7 
