If you do not need to link packages together during the installation, you can use a query that find the machines that have that file AND / OR regKey and any other attribute that is reported already in the inventory. Then use the task to run against the query.
Have a look at this http://community.landesk.com/support/message/1400 how to scan for a specific key
If you want to be more "elegant" you can also use Security and Patch Managers Custom Definition to detect and deploy the software.
Hope this help
When I need to build packages like these, that can't be handled by LANDesk inventory queries, I will use a batch file or script to control the logic of the installation. The batch/script will first check for whatever conditions, and then take the apporpriate actions.