What operating system?
Most likely Windows, XP or 2000
Go to Control Panel>Network Connection. Right click on your network card, select Properties, double click TCP/IP, go to advanced, in the IP setting, you see IP addresses, click add, and add your second IP.
The order of the list matters, so what's on top is used first. I don't see a reason why you need two IP on one NIC. If it's static, it's going to use that one above forever. Unless it was taken by another computer or a conflict happens, then I guess Windows will some how use the second one..eh