My macbook is running slowly. HELP!?

OK. I love hilarious videos. I wanted to download four mp4 vids on my macbook. only like maybe 10 mins of vid, maybe 15. I put it on a flash drive and then deleted them from my computer to save space. Hunky Dory. Maybe the next day, well, im not sure how much later, everything is slow. I have a fast internet connection, which is still fast on the other computers, all new macs. I ran maintenance 3.8, and repaired permissions. NOTHING. Now im runnin somthin called onyX and i hope it works. I need genius halp here people DX