OK, Well in XP Pro you have these users:
Administrators have complete and unrestricted access to the computer/domain
Backup Operators can override security restrictions for the sole purpose of backing up or restoring files
Guests have the same access as members of the Users group by default, except for the Guest account which is further restricted
Network Configuration Operators: Members in this group can have some administrative privileges to manage configuration of networking features
Power Users possess most administrative powers with some restrictions. Thus, Power Users can run legacy applications in addition to certified applications
Remote Desktop Users Members in this group are granted the right to logon remotely
Replicator Supports file replication in a domain
Users: Users are prevented from making accidental or intentional system-wide changes. Thus, Users can run certified applications, but not most legacy applications
Debugger users can debug processes on this machine, both locally and remotely
Help Services Group: Group for the Help and Support Center