%windir%\system32\logfiles\w3svcXXX on IIS 6
C:\Inetpub\Logs\Logfiles\w3svcXXX on IIS 7
These are the default paths.
Also look at c:\windows\system32\logfiles\httperr for 503 and 400 type hits.
Steve Schofield
Windows Server MVP - IIS
http://weblogs.asp.net/steveschofield
http://www.IISLogs.comLog archival solution
Install, Configure, Forget