Note: Plesk has builtin tools to do this for you. Use the Plesk interface to add IPs if possible.

To add aditional IPs to a Windows 2003 server, browse to Start >> Control Panel >> Network Connections >> Local Area Connection (properties).

Select Internet Protocal (TCP/IP) and click properties >> Advanced.

From here you will need to select Add in the IP addresses section and enter in the IP address and Subnet mask for each IP you wish to add to the server.

Once this is complete, simply "OK" back to the desktop.

To verify that your settings have taken effect, open a dos prompt by browsing to Start >> Run >> 'cmd' and run the command

#ipconfig /all