SLOW MOTION ISSUES EVER SINCE

Hey everyone, i've been playing LoL on a 2010 Compaq dx2400 that was bought from my mom's work after about one year of use. Ever since I can remember, it has been really laggy in gameplay. Like for example, when someone would attack me I wouldn't be able to counterattack them because everything would be moving in very slow motion. I even stopped playing for about 2 months due to frustration...haha but I couldn't resist after playing it again at a friend's house. It may help to know that this computer's software was downgraded to XP from Vista by my mom's company. I'm not really sure how to restore everything without completely resetting my computer and I don't want to take chance. Thanks guys.