My logon script are a bit different:
net time \\server /set /yes
net use u: \\server\public
so if you want to map a unit for home's dir
net use z: \\server\home\%u
For "net time" is maybe possible that the xp machines are set to obtain the time by a time server (this setting is on time settings in control panel)?
I hope what I say is ok and an help for you...excuse me for poor english.