Creating site collection templates

1. To add the template to central administration site templates you will need to use stsadm. I recommend that you add the path to the bin directory containing stsadm.exe to the system environment variables.

1. C:\source\site-templates>stsadm -o addtemplate -filename “project-simple.stp” -title “Project site – simple” -description “Project site template with a simple structure”

Operation completed successfully.
IIS must be restarted before this change will take effect. To restart IIS, open a command prompt window and type iisreset.

Source: http://blog.gavin-adams.com/2007/07/03/creating-site-collection-templates/