FreeNAS is basically just a distribution of FreeBSD with programs like Samba set up nicely and a web interface for people who avoid the command line. I'd bet you could use FreeNAS as a print server if you wanted, but you might have to use the command line instead of the web interface to configure that. I haven't used FreeNAS specifically, but I have a FreeBSD server as my home server and it's worked great for me.
Definitely looks like you should be able to use this with Windows clients. CIFS (Samba) is what you want to look at. FTP should work on any platform (that has an FTP client).
I've never used a Biostar motherboard (ASUS in my current workstation), but Athlon 64 processors have worked great for me. Looks like a good home server though.
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