If your primary want in your home server is automated backups of your computers, then, well, use Windows Home Server.
If you don't care about that, and just want to serve files up, XP Pro works well. (that said, WHS makes it simpler - access controls are a lot easier to deal with.)
It's not like WHS is expensive, though - it's, what, $50 for the OS? That's a steal, I'd say.
User logons: in the sense of access controls to the shares, yes. In the sense of 'domain logon', no. WHS 2011, while it's a variant of SBS 2011, has no domain funtionality.
Roaming profiles: again, no (see above.)
Go read about it. Start with the official page
. Google for more.
Me, I'm still rocking my WHS v1 box (sometimes referred to as "WHS 2008.") Still works. Still backs up my machines (which is a godsend - got to take advantage of that again last week, when the kid's desktop HDD died), still stores a lot of random crap for me.
I do want to upgrade to WHS 2011 (or, maybe migrate the home domain to SBS 2011 Essentials.) Either way, need new hardware first....