This Log Parser query assumes that the logs are in c:\logs\web and that IIS has been configured to log in Microsoft's W3C extended log format.  With modified syntax this query could be used to analyze log files in other formats.

The query can be run from a DOS prompt within the Log Parser installation folder.  To save excessive typing or pasting into command prompt windows the whole query can be saved as a .bat file.

logparser "SELECT [cs-uri-stem], [cs-uri-query], Count(*) AS [Hits]
FROM c:\logs\web\ex*.log
WHERE sc-status = 500
GROUP BY [cs-uri-stem], [cs-uri-query]
order by [hits], [cs-uri-stem] DESC" -rtp:-1 -i:iisw3c

Once the query has been put into a batch file (Find500Errors.bat), it can be run using the following when typed at the command prompt (which will produce an output file called 500Errors.txt).

Find500Errors.bat > 500Errors.txt