my 500Mhz AMD K6-2 can play youtube videos fine... for a modern machine to be slowed down to that point is definitely unusual.
Since it doesn't occur in Windows XP or Windows Vista, it obviously isn't "one of those things" that you can't prevent. There must be something different betwee nthe windows 7 and XP and Vista installs.
Do you have all your Drivers installed? Namely Video/Sound?