A great feature of Android is the ability to override many of the default applications for core features of the phones. You are free to pick Skyfire for browsing, DoubleTwist for music playing, and Handcent for text message sending. You can even use completely new launchers like LauncherPro or ADWLauncher. Doing this can leave you puzzled if you want to go back to using the "stock" services and we are here to help.
A built-in feature of Android is there for you in situations just like this. Go into Menu > Settings > Applications > Manage Applications and find the application whose defaults you want to remove. For instance: If you want to go back to your stock launcher after using LancherPro, find the LauncherPro app on the list and click on it. Scroll down a bit and click the "clear defaults" button and you will be set. Next time you do an action that you had set that app as the default for, Android will pop up a notification offering a list of apps that can be set as the default for that action. Select the "Use by default" check box, make your choice, and you are done.