In Windows 2000, the usual method of viewing a file--double clicking on it--doesn't work for BLG files and I couldn't find any information on the 'Net telling you how to open one. Here's how.
Go to Control Panel, Administrative Tools. Open Performance. On the toolbar, the fourth icon from the right looks like the universal hard disk symbol. The tooltip, so helpfully, says, "View Log File Data." Translated into the English you and I speak, that means, "Open a stinkin' BLG file!" BLG is the file format that Performance Monitor uses to save performance data.