If you want to protect a Server running Windows 2003 SP2 or SBS you need to ensure that the correct KB is applied as you will get the following error when trying to select a source.

One or more prerequisites for protecting this data source is missing.
Ensure that the following prerequisite software is installed on server and/or all nodes in the cluster:
Windows Server 2003 SP1 with Knowledge Base article 940349 (http://go.microsoft.com/fwlink/?LinkId=99034)
– OR –
Windows Server 2008 (http://go.microsoft.com/fwlink/?LinkId=128812)

To fix this you need to download and install the KB940349. Once you apply this KB to your  Windows 2003 Server and have rebooted, DPM will then allow you to select a source.

Hope it helps.