I suggest Windows Server 2003 for your server operating system, it can be configured to have each user's "My Documents" on the server as well as a "U:\" drive for all their important files in case of computer crash. It can also handle all of their emails through Exchange.
Also, Acronis Backup and Recovery 10 for backups.
AVG antivirus is a good solution, relatively cheap also.
Not sure about firewall.