Everyone in this thread will probably hate this idea but...
Personally I would not bother with a server as your listed requirements are:
1. Storing files centrally
2. Sharing files (between 10 computers)
3. Sharing a printer
4. Having a Calendar
For your file storage and sharing needs I would custom build a PC with Windows 7 Professional or Ultimate, as Windows 7 supports 20 SMB connections as apposed to the 10 of XP. For storage I would get a motherboard that supports RAID 5 and build an array large enough for your needs (keep your OS on a separate drive). Backups can be made on external drives with Memeo autobackup premium, possible in conjunction with an online solution (though I would not want a third party to have access to all of my files).
My main reason for using a regular desktop is you are not running any server apps. Therefore there is no need to deal with the headaches of a server such as DNS and all the other settings (which are difficult for the novice without a lot of study), the much higher expenses (OS, licenses, Anti-Virus).
As for the shared printer. Just buy an HP with built in Ethernet. Of all the brands I have set up HP is by far the easiest to configure.
For your calendar just use Google or Yahoo calendar. They are free and work great. I found Google calendar to be easier to configure. Plus you can set up G-Mail and relay through their system so that you have free IMAP support lots of e-mail storage space and still pipe everything through your domains e-mail when configured correctly (to avoid the @gmail.com on your sent e-mail).
Other things you may want to consider. For networking I would set up a multiport gigabit switch then connect that to a router. If you need remote access to your server you can use gotomypc.com's services. For anti-virus you can use Avast-Antivirus (their business edition is $50 but much better than Norton or McAfee). For the backup I would use
Memeo Professional Backup as mentioned above. Then you can keep two backups and rotate them out daily or backup to a local external drive and to an off site device via FTP.