There are number of ways to download Youtube videos directly to your computer for viewing it offline.
But I found the following method to be most effective.
Step1:Browse for the video you want to download.
Step2:Get the link for the video. The links are generally of this kind- http://youtube.com/watch?v=CJteb1lXFig
Step3: Copy the URL and put it in any of the following Sites to download the raw file.
Step4: After downloading the file, rename the file’s extension to .flv
Step5: Use any of the Video Players from the link below to play your FLV file.