Saying "... trying to register its copy as an autorun startup object" means that the program is trying to put an entry in your Start Up folder so that when you initially start each session of Windows, then the Adobe Reader is loaded into memory ready to be instantly called up when needed. Your security software must be set to warn you about this and there should be provision to reject the start up entry but still continue installing the program proper.
It is not necessary to have it in your start up folder. If you have Adobe Reader on your computer, when a program needs A R to start, it will just fetch it from the program files and run it then.
The problem that most people have with slow start up of Windows is that they have several programs that have nested themselves in the Start Up folder so that they AutoRun and the act of starting them up delays the start of Windows.
If you download CCleaner (Google will find it) and click on Tools and then StartUp it will show you which programs you have in the startup folder and allows you to disable them from starting automatically. Most people only need their Security software and their Network software to be in that folder. You can disable them with CCleaner and, if anything is then wrong, you can re-enable them one-by-one until the problem is cured.
It will make a big difference to your delays on start up if you find you have about a dozen active files in that folder.