Why Is My Computer Running Slow

I download alot of videos from youtube and other places and now my computers is really slow ( but only on the internet ) and i was wondering is having alot of videos would slow it down?
Also if its not the videos i just download DivX ( maybe thats the problem? ) right before it started getting really slow, do you think that could make it really slow?

P.S. its not my internet connection because all the other computers in the house run normally.

# How I deal with the days where “nothing gets done” # Partition Magic and Partition Logic # Manipulating boot.ini # Wireless network software # Craigslist tips # Customer complaining of overcharge