Server won't restrict access to XP Machine

I have W2K server set up as a domain controller. I use my personal computer
(WinXP Pro) to log into the server, so my computer is obviously on the
domain. My wife's computer is an XP Pro machine, and it isn't on the
domain, only on the Workgroup named "workgroup." On the server, I have a
hard drive which I have enabled Sharing on, but only to Domain Admins. The
security is also set to Domain Admins only. For some reason, the server
will not restrict access to my wifes computer. She is an admin on her
computer, because she is the only user, and XP Pro forces the only user to
be an admin. But as I see it, this should make no difference on the server
side, which is domain related. Why will the server not lock out "Workgroup"
users? Is this some type of security flaw? Thanks for any suggestions.

Brian