If you have been testing other CR schedulers, or using other CR related products then some of them come with the Developer components and dlls bundled in them. So CRD would have used these components too. When you uninstall the other products, they uninstall the components and CRD can't find them and stops working.
Installing CR developer ensures that the components are on the PC whether other products are installed or not.
You can see more about this by looking at http://www.christiansteven.com/crdprereq.htm
