What is happening to my computer!?!?

Please help!! When I turned on my computer the login name and pics were HUGE and so was the "welcome" When i logged in it was the same thing on my desktop so i went to properties>settings> and then i changed it. I fixed it. But why was it like that? Then i opened up internet explorer and whenver i scroll, it is slow and its like a wave across the screen! My computer seems to go quickly but when i scroll it seems slow. Now my computer is slow after using it for a while. I went to Tools> Internet Options> Advanced> and restored it to the original settings and it fixed the scrolling for only like 2 minutes. After all of this, now my computer screen went black quickly and then it came back slowly with black lines all over horizontally on the screen. Im scanning with Marwarebytes but so far doesnt detect anything! ALSO at the beginning, when i barely logged in, a Windows File Protection box came up and says: "Files that are required for Windows to run properly must be copied to the DLL cache Insert your Windows XP Proffesional Service Pack 3 CD now." Yesterday i had downloaded a format converter that someone from Yahoo Answers had told me to download. I downloaded it, but didnt install it all the way because it didnt look like something i could trust. Could this be the problem? The Windows File Protection box started showing up before though.... SO SORRy for all the text but i REALLY need help !! THank YOU!!