Asrock octa-core...running windows server?

Hey all, I re-watched the video of Wendell reviewing the asrock board and I think it would be perfect as a small business server, around 4 workstations, for my parents home business.

I must warn you, I'm a complete novice with servers/software but I have built several gaming PCs.

Their current filesharing  setup is pretty prehistoric, a pentium 4 IBM Pc with a measly 1gb of ram and a 2tb hdd running XP pro. It sits at 100% CPU and ram throughout the day and will probably fail in a few months.

Currently all the workstations do not have access to the internet but with this new setup I'd like to create modified user accounts using windows server? And purchase some sort of anti virus so that no malware etc has a chance to corrupt the files stored on the server.

Regarding the new hardware, I'll saturate the dimm slots with ecc ram and have around 10-15tb storage space

What I want to achieve with the server:

1. Store modified user account settings, passwords, filter lists for websites etc

2. Act as a file server 

3. Backup data to nas drive

Just by metyping this out I can see that this is going to be a huge task :o. What are your thoughts for me using windows server? 

Thanks guys!