Clearing SQL Server 2005 event log
Log files in question are named ERRORLOG, ERRORLOG1 etc. and are usually stored in LOG folder somewhere inside MSSQL installation directory. In the beginning of the log there is a message like this:


  Logging SQL Server messages in file 'C:\Program Files\Microsoft SQL
  Server\MSSQL.1\MSSQL\LOG\ERRORLOG'


It can help to determine the exact path.

If the need to delete these logs is caused by a security concern, it's good to execute sp_cycle_errorlog proc to make log cycle. This way the ERRORLOG file will be empty and all the events will be in ERRORLOG1, ERRROLOG2 etc which can be deleted at any time.