I assume that your reference to 'downloading videos from Youtube' (rather than to simply watching them on Youtube) means that you're saving the videos to your hard drive and then viewing them in a media player.
So the obvious question has to be 'which player?. If it's the dreaded (resource-hungry) Windows Media Player, that might account for your problem. Try using a decent player instead, such as the much-lauded VLC
http://www.videolan.org/vlc/download-windows.en_GB.html
or the lesser-known (but still excellent) GOM:
http://www.gomlab.com/