There are 2 kinds of load balancing in SharePoint 2007:
- Load balancing for web requests
- Load balancing for applications
Load balancing for web requests is not included in SharePoint, you should deploy a network load balancing solution. This can either be hardware or software like the feature available for Windows Server. It envolves a virtual IP address (VIP) that is mapped to the load balancer via DNS. Web requests are then spread over the web servers.
Load balancing for applications is a different kind of load balancing. It is done internally by SharePoint. There is no configuration possible, it uses a kind of round robin mechanism for balancing requests between application servers.
There are essentially 2 common applications that can be load balanced: Excel Services and the Search Query role. The Search Index role is a single point of failure as it cannot be deployed on multiple servers (in SharePoint 2010 this is possible).
You enable load balancing for applications by activating the services on more than one server. SharePoint then takes care of it for you.
If you want to find out if Windows Network Load Balancing is installed on your web servers, go look for the Network Load Balancing Manager in your administrative tools. A very good introduction on how to configure this can be found here.