IIS 7.0 logging is NOT enabled by default. You must add this as an additional role service under Health and Diagnostics > HTTP Logging.

Default path to logs is: %SystemDrive%inetpublogsLogFiles

Error Codes (taken from here http://support.microsoft.com/kb/943891)

1xx – Informational
These status codes indicate a provisional response. The client should be prepared to receive one or more 1xx responses before receiving a regular response.
100 – Continue.
101 – Switching protocols.

2xx – Success
This class of status codes indicates that the server successfully accepted the client request.
200 – OK. The client request has succeeded.
201 – Created.
202 – Accepted.
203 – Non-authoritative information.
204 – No content.
205 – Reset content.
206 – Partial content.
 
3xx – Redirection
The client browser must take more action to complete the request. For example, the browser may have to request a different page on the server or repeat the request by using a proxy server.
301 – Moved Permanently
302 – Object moved Temporarily
303 – See Other
304 – Not modified.
307 – Temporary redirect.
 
4xx – Client Error
An error occurs, and the client appears to be at fault. For example, the client may request a page that does not exist, or the client may not provide valid authentication information.
400 – Bad request.
401 – Access denied. IIS defines a number of different 401 errors that indicate a more specific cause of the error. These specific error codes are displayed in the browser but are not displayed in the IIS log:
401.1 – Logon failed.
401.2 – Logon failed due to server configuration.
401.3 – Unauthorized due to ACL on resource.
401.4 – Authorization failed by filter.
401.5 – Authorization failed by ISAPI/CGI application.
401.7 – Access denied by URL authorization policy on the Web server. This error code is specific to IIS 6.0.
403 – Forbidden. IIS defines a number of different 403 errors that indicate a more specific cause of the error:
403.1 – Execute access forbidden.
403.2 – Read access forbidden.
403.3 – Write access forbidden.
403.4 – SSL required.
403.5 – SSL 128 required.
403.6 – IP address rejected.
403.7 – Client certificate required.
403.8 – Site access denied.
403.9 – Too many users.
403.10 – Invalid configuration.
403.11 – Password change.
403.12 – Mapper denied access.
403.13 – Client certificate revoked.
403.14 – Directory listing denied.
403.15 – Client Access Licenses exceeded.
403.16 – Client certificate is untrusted or invalid.
403.17 – Client certificate has expired or is not yet valid.
403.18 – Cannot execute requested URL in the current application pool. This error code is specific to IIS 6.0.
403.19 – Cannot execute CGIs for the client in this application pool. This error code is specific to IIS 6.0.
403.20 – Passport logon failed. This error code is specific to IIS 6.0.
404 – Not found. 404.0 – (None) – File or directory not found.
404.1 – Web site not accessible on the requested port.
404.2 – Web service extension lockdown policy prevents this request.
404.3 – MIME map policy prevents this request.
404.4 – No Handler (IIS 7)
404.5 – Request Filtering: URL Sequence Denied (IIS 7)
404.6 – Request Filtering: Verb denied (IIS 7)
404.7 – Request Filtering: File extension denied (IIS 7)
404.8 – Request Filtering: Denied by hidden namespace (IIS 7)
404.9 – Denied since hidden file attribute has been set (IIS 7)
404.10 – Request Filtering: Denied because request header is too long (IIS 7)
404.11- Request Filtering: Denied because URL doubled escaping (IIS 7)
404.12 – Request Filtering: Denied because of high bit characters (IIS 7)
404.13 – Request Filtering: Denied because content length too large (IIS 7)
404.14 – Request Filtering: Denied because URL too long (IIS 7)
404.15- Request Filtering: Denied because query string too long (IIS 7)
405 – HTTP verb used to access this page is not allowed (method not allowed.)
406 – Client browser does not accept the MIME type of the requested page.
407 – Proxy authentication required.
412 – Precondition failed.
413 – Request entity too large.
414 – Request-URI too long.
415 – Unsupported media type.
416 – Requested range not satisfiable.
417 – Execution failed.
423 – Locked error.

5xx – Server Error
The server cannot complete the request because it encounters an error.
500 – Internal server error.
500.12 – Application is busy restarting on the Web server.
500.13 – Web server is too busy.
500.15 – Direct requests for Global.asa are not allowed.
500.16 – UNC authorization credentials incorrect. This error code is specific to IIS 6.0.
500.18 – URL authorization store cannot be opened. This error code is specific to IIS 6.0.
500.100 – Internal ASP error.
501 – Header values specify a configuration that is not implemented.
502 – Web server received an invalid response while acting as a gateway or proxy.
502.1 – CGI application timeout.
502.2 – Error in CGI application.
503 – Service unavailable. This error code is specific to IIS 6.0.
504 – Gateway timeout.
505 – HTTP version not supported.