Is this what you are talking about?
How to install the Microsoft Loopback adapter in Windows XPhttp://support.microsoft.com/kb/839013
If it is, why did you need that?
To configure TCP/IP for static addressing
Open Network Connections.
Right-click the network connection you want to configure, and then click Properties.
On the General tab (for a local area connection) or the Networking tab (all other connections), click Internet Protocol (TCP/IP), and then click Properties.
Click Use the following IP address, and do one of the following:
For a local area connection, in IP address, Subnet mask, and Default gateway, type the IP address, subnet mask, and default gateway addresses.
For all other connections, in IP address, type the IP address.
Click Use the following DNS server addresses.
In Preferred DNS server and Alternate DNS server, type the primary and secondary DNS server addresses.
To configure advanced static address settings for a local area connection, click Advanced, and do one or more of the following:
To configure additional IP addresses:
On the IP Settings tab, in IP addresses, click Add.
In TCP/IP Address, type an IP address in IP address and a subnet mask in Subnet mask, and then click Add.
Repeat steps 1 and 2 for each IP address you want to add, and then click OK.
To configure additional default gateways:
On the IP Settings tab, in Default gateways, click Add.
In TCP/IP Gateway Address, type the IP address of the default gateway in Gateway. To manually configure a default metric, clear the Automatic metric check box and type a metric in Metric.
Repeat steps 1 through 3 for each default gateway you want to add, and then click OK.
To configure a custom metric for this connection, on the IP Settings tab, clear the Automatic metric check box, and then type a metric value in Interface metric.
To open Network Connections, click Start, point to Settings, click Control Panel, and then double-click Network Connections.
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings might also prevent you from completing this procedure.