If you need to have the logs in a folder in XML, elmah-loganalyzer might be of use.
You can also use Microsoft's Log Parser to perform "sql like" queries over the xml files:
LogParser -i:XML "SELECT * FROM *.xml WHERE detail like '%something%'"
EDIT:
You could use a combination of nutch+SOLR or logstash+Elastic Search as an indexing solution.
- http://wiki.apache.org/nutch/NutchTutorial
- http://lucene.apache.org/solr/tutorial.html
http://blog.building-blocks.com/building-a-search-engine-with-nutch-and-solr-in-10-minutes
- http://www.elasticsearch.org/tutorials/using-elasticsearch-for-logs/
- http://www.javacodegeeks.com/2013/02/your-logs-are-your-data-logstash-elasticsearch.html