How to configure Usage Analysis Processing on a Web server that is running Windows SharePoint Services 3.0

To turn on Windows SharePoint Services usage logging in an Office SharePoint Server farm, you must first turn on Usage Analysis Processing in Windows SharePoint Services 3.0, and then turn on Advanced Usage Analysis Reporting service in Shared Services Administration. To do this, follow these steps:

1.Click Start, point to All Programs, point to Administrative Tools, and then click SharePoint 3.0 Central Administration.
2.On the Central Administration page, click Operations.
3.On the Operations page, under Logging and Reporting, click Usage Analysis Processing.
4.In the Logging Settings section, click to select the Enable logging check box.
5.In the Log file location box, type the location where you want to store the log file.The default location for the log file is \Program Files\Common Files\Microsoft Shared\Web Server Extension\12\Logs.
6.In the Number of log files to create box, type a number between 1 and 30. In general, use a number that is one to three times the number of database servers in your server farm, with a maximum number of 30 log files.
7.In the Processing Settings section, click to select the Enable usage analysis processing check box.
8.Under Run processing between these times daily, specify the range of times to start the usage analysis log processing.In the Start box, select the earliest time of day to start running log processing. In the End box, select the latest time to start running log processing.
9.Click OK.

Refferance:-
http://support.microsoft.com/kb/825541