Boot holding the Shift key. That will boot you into Safe Boot mode. Many startup items will be bypassed, as well as some of the drivers.
Check in your System Preferences/Users & Groups pane, then the Login Items tab. Remove items that you don't want starting with your Mac user. (Delete items, don't just uncheck the box.) Nothing is actually removed, and you simply find those apps to use them when you need to.
Can you tell us what kind of problems you are having? System really slow? Do you see the spinning beach ball (often?)