Maximum PC

This is a bit of a strange question, but can WinXP Pro be used in place of WHS in terms of creating a small home file server?

Does WHS offer things that you would find a true Server package, such as user log ins, network shares, roaming profiles ,etc. I've never really used WHS first hand... only read about it. However, I have used Windows Server 2003 a bit, and am familiar with it. Just curious if WHS gives me similar results. I mainly want my server to be able to back up my machines' data into a single solution.

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.

WHS and...
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....