http://blog.bennett-scharf.com/2008/12/17/converting-an-existing-perfmon-blg-file-to-csv/
http://blogs.msdn.com/adcman/archive/2006/05/15/598149.aspx
Example syntax for simply converting between file formats:
relog -f csv inputfile.blg -o outputFile.csv
And then the CSV can be used as input to LogParser.
http://blogs.technet.com/b/askperf/archive/2008/05/20/two-minute-drill-relog-exe.aspx
More sophisticated filter options with relog.exe
