Guest account need & requirement in SQL Server
Although the guest user remains in all of the user and system databases performing the same general functionality in the SQL Server 2000 and previous versions, at the same time you can revoke the access as mentioned in BOL http://msdn2.microsoft.com/en-US/library/ms177573.aspx link. In the previous versions including SQL 2005 it is always needed in the master and tempdb databases. Also when Log Shipping is used with SQL Server Enterprise Edition. To get information about existence of this account:
EXEC sys.sp_MSforeachdb 'SELECT * FROM sysusers'
GO