sp_cycle_errorlog

Closes the current error log file and cycles the error log extension numbers just like a server restart. The new error log contains version and copyright information and a line indicating that the new log has been created.

Every time SQL Server is started, the current error log is renamed to errorlog.1; errorlog.1 becomes errorlog.2, errorlog.2 becomes errorlog.3, and so on. sp_cycle_errorlog enables you to cycle the error log files without stopping and starting the server.

http://technet.microsoft.com/en-us/library/ms182512%28v=sql.100%29.aspx