You can upgrade from SQL Server 2000 or SQL Server 2005 to SQL Server 2008 using either an in-place or side-by-side upgrade.
Prior to upgrading, run the Upgrade Advisor, available at http://www.microsoft.com/sql, against your SQL Server 2000 and 2005 instances and code base to determine any incompatibilities.
An in-place upgrade allows you to install SQL Server 2008 over the top of an existing SQL Server 2000 or 2005 instance. SQL Server will perform the following actions during an in-place upgrade:
Install SQL Server 2008 binaries
Upgrade the SQL Server 2000 or 2005 databases
Remove SQL Server 2000 or ...