NSD is not an error. It is a diagnostic. The result of the diagnositic is a file that starts with "nsd" and ends with ".log." You will find the file in the IBM_TECHNICAL_SUPPORT folder.
A previous respondent was correct, you should upgrade, but you should upgrade to the latest available fix pack for your version, which is v6.5.6.
If NSD runs again due to an error after the upgrade, you can download the Lotus Notes Diagnostic analyzer here:
Use that tool to find out more information before calling IBM support, or simply call IBM support and provide them the NSD file.
In the previous answer there was a reference to disabling spell check and installing the SAP front end. However, there is no mention of spell check being involved in the error, or that you have an application that interfaces with SAP. I have removed those references, but they can be found in the answer history.
Finally, end of life support for Lotus Notes v6 has been announced. After April 30, 2010, there will be no technical or engineering support for that release. Also realize that there have been three releases of Lotus Notes since v6.5 -- v7 (a minor release from the client perspective), as well as v8 & v 8.5, which have provided powerful new capabilities.
<a href="http://www-01.ibm.com/support/docview.wss?uid=swg21287154">v6.5 EOS</a>