You might run into this with a QIP-based DHCP as a backend with XP SP3 clients: Some DHCP Options are not recognized on a Windows XP SP3-based client computer when the DHCP server offer includes option 43. Install KB 953761.
Long story, short: Due to the addition of vendor specific options of the XP SP3 DHCP client request, relating to NAP, the code expects DHCP options to be returned in a certain order. When paired with a MS-based DHCP server, correct behavior is observed. If used with a different DHCP server that might reorder the DHCP reply, the original XP SP3 DHCP client code might not correctly set all DHCP settings.
There is a two page thread on the TechNet forums about this issue here.