Windows clients not using NTP server provided via DHCP
Solution 1:
The registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dhcp\Parameters\Options\4
indicates that Windows doesn't actually make use of the required option that would set the NTP server (DHCP option 42). The Microsoft Support article DHCP Options Supported by Clients indicates that adding additional keys here will not have any effect.
Thus, it seems to me that it is simply not natively supported to set NTP servers via DHCP on Windows Vista/7.