I have deployed a third part webservice on my webserver, this service throw its errors to the windows error viewer, but those error has never been wroten, the webservice shows this error: Cannot open log for source {0}. You may not have write access, I've checked out the Error log configuration in the error viewer and I've asigned full permission to the EVERYONE user group on the log file folder. the error still been appearing, I don’t understand why, if somebody had been working arround somthing like this and wants to gives me a hand on this will be nice.

Log writting Exception thrown by a WebService
subhanet
See http://support.microsoft.com/kb/323076
I would also contact the third party vendor and get their recommended solution.
csname
ClaudiaHelpOnVSTO