Can't change static IP address to dynamic

[bingbong]

I've got a weird problem I can't get my head round. I have a fairly standard XP machine with a static IP address, however I now want to change it to a dynamic address. So I untick the use the following IP/DNS address boxes from the TCP/IP properties pages in network connections. The machine then wants to be rebooted, so I reboot, but on restart the settings are back to what they were. Am I missing something silly, I've rebooted about 6 times now.
Any ideas?

[bingbong]

I using the menu that Lord Grover posted, It has fixed IP address in all the boxes. I change it so it looks exactly as above by clicking the radio boxes which clears the IP's. I click OK, and it says computer must be restarted, I restart - and all the boxes are filled in again!