sp_cleanupwebtask is no longer supported in SQL Server 2008. SQL Server 2005 supports the following three undocumented web task system stored procedures:
The web task system stored procedures are not enabled upon installation. To enable these stored procedures, you should use the Web Assistant Procedures Option to enable the Web Assistant procedures on this server.
To use the Web Assistant, you must also run the Web.sql script, available in the SQL Server 2005 installation location, in the MSSQL.x\MSSQL\Install folder.
Use Reporting Services instead.