When you configure a new VM you have the option to add a "Network Adapter" or a "Legacy Network Adapter". Wich one should you use?
The Legacy Network Adapter emulates a real Hardware NIC and chances are good that your guest already has a driver and can use it right away. It is also the one that supports PXE Boot, as the most PXE Loaders support this real Hardware.
The default Network Adapter is a synthetic device not found as real hardware. It is much faster, however it requires drivers that are usually not included in any OS today. You need to add them by installiing the Integration Components (IC) on a supported Operating System. For Windows these ICs can be easily added via Action->Insert Integration Services Setup Disk inside the VmConnect Console.
For SLES 10 see http://blogs.msdn.com/virtual_pc_guy/archive/2008/09/22/rtm-of-linux-integration-components-for-hyper-v-now-available.aspx