@Walter Hinteler,
1. Go to services (Start->All Programs->Administrative Tools->Services) (Alternately, Start->type "services.msc"->Enter/Search)
2. Stop the SQL server service (SQL Server->Right click->Stop)
3. Disable the SQL server service (SQL Server->Right click->Properties->Change "Startup type" to "disabled"->OK)
Run this way for a while (several days at least) and see whether all of your applications continue to work.
If everything works, then uninstall the SQL server. If something breaks, then change the startup type back to automatic and start the service, then we can troubleshoot why the SQL server service does not start.