i hope you havent formatted the drives
kindly post your specs so we can help better
now these are the things you can do (you should check that your hard isnt the cause if the playback sound (audio) is chunky and video is playing way too slow you have a hdd with bad sectors it happens sometime especially with western digital hard)
1. a fresh installation never slows the pc speed so you might have downloaded or installed something like a trojan or malware for the solution try this
0000000000000http://thepiratebay.se/torrent/6919425
2. now if u r still facing problems u need to boot from a live cd like hiren but as u r not computer literate ask some friend to do it
3.you might have low ram or low space on C
update us if nothing works