Regarding the H: assignment, you may find that Z: and or H: have already been assigned by the server. If you've got persistant connections, the H: will be used for profiles, therefore stopping you from trying to change or renew the mapping.
As for the NET TIME \SERVER /SET /YES, it does work on 2000Pro and XPpro, provided that you have turned off secure logins (end of the XP & SMB 2.2.2 FAQ)
It fails when I'm not a domain member, or I'm logged into a different domain though
Regards,
Craig Foster