What version of OS X are you using? 10.6 (Snow Leopard), 10.12 MacOS Sierra, or something else? It would help to know what you're using.
First thing I'd do is remove all items from Login Items and Startup Items.
1. Login Items are in System Preferences > Users & Groups > tab is located right next to Password
2. Startup Items are in boot disk (usually called Macintosh HD unless you changed it) - /Volumes/(boot disk)/Library/StartupItems
You'll need your admin password to delete the files.
You can also go into LaunchAgents and LaunchDaemons and delete those items. They are in three places:
1. /Volumes/(boot disk)/Library/LaunchAgents
2. /Volumes/(boot disk)/Library/LaunchDaemons
3. /Users/(your home folder name)/Library/LaunchAgents
You can easily get to #3 by launching Finder, click on Go on the menu bar up top, hold Option key and click on Library that will pop up. If you're on an older OS like Snow Leopard, Library will already be showing.
Some of the launch agents and daemons you might want to keep, rather than deleting all of them. If you have Microsoft Office installed, I'd leave com.microsoft.office.licensing.helper.plist alone, for example.
If you're on a newer OS, you should run Malwarebytes, too. It might find something lame in there causing you problems. Free for Mac:
https://www.malwarebytes.com/mac/
Follow this help file, too:
https://support.apple.com/en-us/HT201262
