It could be any one of a number of reasons. And it varies for different types of video. For example, video over Skype or other instant messaging services often seems to be jerky and unsynchronised with audio, no matter how fast your computer is. But low res videos on Youtube should be ok. I have a fast machine and sometimes BBC iplayer sticks a bit, but is watchable, and Skype always sticks (which makes my family look hilarious!)
Firstly, close any other programs that are using RAM and slowing down your machine.
Close a few tabs in your browser - (if you're like me I always seem to have at least ten open at once) or pages - especially on other sites that have lots of graphics or video.
If no joy, clean your system CCleaner.com is a good free tool that will clean unused files without causing any damage, then defrag.
Remove programs (and unneccesary toolbars you don't use.
Remove programs that don't need to be in the system tray.
Add RAM if you have less than 2mb.
Ensure you always keep MS up to date with auto downloads.
Another handy tip is to go to a site that tells you the CORRECT speed of your internet connection (not what your isp tells you) such as: www.internetfrog.com/mypc/speedtest/ So if you pay for "up to 2mbps" you may not receive that speed all day. Keep track of the speeds throughout the day, and just view video at the times you have the fastest speed!
It might be worth upgrading to a faster tarriff. I have Virgin Media 2mbps which is good most times, but others on 8 - 20 mbps are often wasting money as they only occasionally get faster speeds than me.
If still no joy, do a search on this forum for other ways to make your PC faster, from more experienced peeps than me.