Consider Future Use

• The first question you should ask yourself before deleting an installer is whether you might need it again. If the answer is "no" then delete it. Many installers can be easily downloaded them again from the Internet. They may even be updated. If, however, the installer is particularly large, you may want to keep a copy just in case.
  
  

Corrupted Installations or Files

• Programs don't always install properly, even if they seem to. Before deleting the installer files, run the program you've installed to ensure it's functioning properly. Only delete the installer files once you're confident the program works. In the event it did not install properly, you can uninstall it and run the installer again. If it still doesn't work, download the installer again in case it is somehow corrupted.
  
  

Software May Require the Files

• Some programs need to be installed on your system to work. You simply choose to run them right from within their folder. In this case, the installer files are actually a part of the program, so you can't delete them; doing so will just cause the program to cease functioning.
  
  

When In Doubt...

• Keep the installation files if there is any doubt to their necessity. The worst case scenario is that you end up losing some hard drive space to files you don't actually need. This is preferable to breaking a program you use and then having to reinstall it. If you're hurting for hard drive space, run your operating system's built-in cleanup program. It will inform you of all the files it can safely remove.