Defrag ubuntu precise

I know that ubuntu is not suppose to need defragmenting due to the way it utilize's the hard drive which is why I switched from windows + alot of other reasons, but it seems that since I use bittorrent quite a bit, my movie files are freezeing up and my music files are skipping and freezeing up after so much use. Also it is takeing alot longer to boot up and close down. I do have half my hard drive free so space is not an issue. I also use bleach bit to make sure that I have a clean drive... I tried to follow the instructions on the wiki site to download and install the program for defragging but it does not seem to be as easy for me to follow under precise as it asks me to change certain lines in the programming, and what it shows in the example does not reflect what I see and also It wont give me permission to change any lines... As it is dangerous to mess with this too much is there anything else I can do?