There are many, many things that could cause that.
For example, look at the NewTek TriCaster Studio. It can encode a web stream, record a stream of video, play back two streams of video plus switch 6 live video inputs all at once.
That runs on Windows XP with pretty moderate hardware, but it's a well oiled machine.
There are any number of factors that could cause the video to skip on your system but I can guarantee you that it's not a Windows design flaw.
Think it is, I use the same program for video encoding as it is the best regardless of cost for FLV conversion and Windows ignores any setting of task priority to "low" on Task Manager and happily assigns 99-100% CPU to the encoder process

Also it works just as badly if you try to get Windows to play HD content better by telling it the CODEC process needs to be "high" or "realtime".
It's like those options are fake and don't actually do anything haha