First, read this (Joe Field, CruiseControl.net from Scratch) !  It will help tremendously. 

Stepping cautiously through the config helps a ton.  I recommend downloading the cruisecontrol.net source code, however, so you can track down bugs.

INTERNAL ERROR: Value cannot be null. Parameter name: path

When running ccnet.exe after configuring vss details, I got the error:

INTERNAL ERROR: Value cannot be null. Parameter name: path
----------
System.ArgumentNullException: Value cannot be null.
Parameter name: path
at System.IO.Directory.CreateDirectory(String path)

I googled it to find http://jira.public.thoughtworks.org/browse/CCNET-102 describing the symptom and recommending a correction.

This turned out to be a null Working Directory!  The user that I configured is not a real developer, but a user for automated builds, and has no working directories configured.  I attempted to set path to foler as a config item but it did not correct the issue.  I resolved the issue by setting a working directory on  the target project in VSS.  After I did that, the source control monitor worked!  I then removed that setting and it seemed to persist.  So set it carefully!

Getting Lateset using VssGet (NAnt contrib)

using a target in my nant config simila to this, i get the following error:

   <target name="getsource" depends="clean">
        <vssget  user="autobuild" password="buildit"
            localpath="C:\TestDrectory"
            recursive="true"
            dbpath="\\VSSServer/INIFolder"
            path="$/Project/Source"
            />
    </target>

Failed to open database
The SourceSafe database path autobuild does not exist.  Please select another database.

Hmmm.   Did i not specify that 'autobuild' is the user?  And the path is correct! 

Well, you may just need to include the '\srcsafe.ini' in the path.
    <target name="getsource" depends="clean">
        <vssget  user="autobuild" password="buildit"
            localpath="C:\TestDrectory"
            recursive="true"
            dbpath="\\VSSServer\INIFolder\srcsafe.ini"
            path="$/Project/Source"
            />
    </target>

will get you there