Windows Xp:
C:\WINDOWS\system32\Logfiles\W3SVC1
Windows NT/2000:
C:\WINNT\system32\Logfiles\W3SVC1
Windows 7
C:\inetpub\logs\LogFiles\W3SVC1
About IIS Log file formats – http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/bea506fd-38bc-4850-a4fb-e3a0379d321f.mspx?mfr=true
Tool to analyze IIS log –
http://indihiang.codeplex.com/
http://www.ehow.com/list_6934814_log-analysis-tools-iis.html