To fix the errors right after downloading the Logstash 1.5, update the windows event plugin to get the fix:
C:\logstash\install\bin>plugin update logstash-input-eventlogUpdating logstash-input-eventlog
Updated logstash-codec-plain 0.1.6 to 1.0.0
Updated logstash-input-eventlog 0.1.5 to 1.0.0