First, I would suggest updating your graphics card drivers.
To do so, find out what make it is.
Using start menu > dxdiag (use the search function in WIndows) and looking under "Display", finding your card ,and Googling for the drivers for that specific GPU.
Secondly, close out any programs that you don't need running. (Windows live / Aim / Skype / MSN / Steam) and defragmenting your computer. Your specs of your PC would help a lot, too.
Does it lag on DVDs? Or just videos like Youtube and stuff ON your PC? Because if it's DVDs, the laser eye in your CD / DVD player can be going bad.
Is it an OLD PC? Or relatively new?