One of the SBS 2003 servers we support has had a puzzling issue. On random days, the server will become unresponsive when Windows Backup runs. After restarting the server, all is OK for days, even weeks. What makes this so odd is that there's been no error in the server logs to indicate a problem.
We discovered Microsoft issued a Post SP2 hotfix for this problem and the issue is related to the Volume Shadow Copy Service (VSS) that takes a snapshot of the file system to ensure a complete backup.
Please note that hotfixes have not completed regression testing, so proceed at your own risk. Check out the bulletin at http://support.microsoft.com/kb/949391/en-us.