For Windows XP SP3 and above I use the netsh command. Especially powerful in Vista/Windows7/Server 2008.
To change from DHCP to Static I use this script ..
interface ip set address name="Local Area Connection" source=static addr=10.135.150.210 mask=255.255.255.0
interface ip set address name="Local Area Connection" gateway=10.135.150.254 gwmetric=0
interface ip set dns name="Local Area Connection" source=static addr=10.100.14.249 register=NONE
interface ip add dns name="Local Area Connection" addr=10.100.12.249 index=2
interface ip set wins name="Local Area Connection" source=static addr=none
The script syntax varies with windows versions, mine is for windows 7