This is how to install only SSIS on SQL Server 2008, you may do this if you already have a SQL cluster and something requires SSIS.
Note that you need SQL Standard or Enterprise to do this.
1. Run setup.exe as normal.
2. Select “Installation” and then select “New SQL Server Stand-along installation or add features to an existing installation”
3. Ensure all prerequisites are ok and then press “ok” to continue.
4. Accept the Product key.
5. Accept the EULA.
6. In the “Features” section select only “Integration Services” and “Management Tools” press “Next” to continue install.
7. Accept all the “Next” options to complete install.
8. Reboot after install and now SSIS will work.