To elaborate on david's #4...
When you install software many programs add programs or services into your startup and it takes time to load each of these processes.
If you load autoruns and look at the Logon Tab you will see most of the startup programs. Many things can probably be deleted to speed up your computer but some are needed. Automatic update programs for Adobe, Google, Firefox, HP Printers etc are among the items that can be removed. Always research any item before you disable it and especially make sure it is not part of your anti-virus software which helps protect your computer. And of course if you stop any update programs then you have to take the responsibility to manually check for updates periodically.