windows 7, high cpu usage, more then xp?

Hey, i just got windows 7, previously had windows xp.
I didnt change any hardwares, just installed windows7 and all the updates with it.
On xp i never checked the cpu usage, so i dont know if it was as high as now, but at the moment since i got windows7 it seems to be very high.

When im just browsing its around 10-15%, when i watch youtube, can be around 30%.
And when im watching a full hd movie (1080p), its arounr 50-55% - and whats the worse the movie is laggy, so its not smooth, its seems to lagg and it hurts my eyes.
Now i have definitely not noticed this when i had xp, the hd movie was running just like the others.

I have an AMD athalon II x2 250 processor 3.00GHz
Video card, Ati Radeon HD 4350 1GB.

I know its pretty basic hardware, but as i said whats wierd is that with these, i didnt have the laggy hd movie problem. Any ideas why this is with windwos 7?

Ty for reply!

As for the flash, yep i have the latest version, as i just installed the win7 OS.
It was new to me, but i see about the use of resources, but on another note i have got 4gigs of ram. Thats why i got the 64bit version i think.

About the drivers, i installed the suggested, important ones straight after i installed the OS, it seemed to have found everything, but if theres something i need to find and install manually please let me know.

And to the actual problem, my biggest worry is when im watching full hd (1080p) movies on the computer (not youtube) but dvds, etc. I tried many different players, windows media player, gom player, vlc player, mkv player etc. but the same problem occurs with all of them.
And the problem is that when im watching the movie, the cpu runs at 50-55% (with nothing else running in the background, and the movie is a bit laggy. Its not extremely bad, but it doesnt run smooth as it should and after 10minutes of wathcing it like that, my eyes strats to hurt. This is the whole reason i decided to get some help, its unwatchable.

And whats causing me the biggest confusion, and problem is that i never previously experienced this on windows XP. As i said i had the same setup, and watched full hd movies all the time, so i dont even have the slightest clue of whats changed

Yep i had a look but theres no marked with exclamation points or red marks... :S
And also theres no suggestions at the windows updates section.

Thanks you for the link! I have downloaded and installed it.
But it didnt resolve my problem
I started to think it was the movie file that was corrupt, but tried a different movie and thats the same as well.
This problem only occures whilst im watching full hd, 1080p.
If im watching 720p movies, they are fine. :S

To me it seems if my graphics card wasnt good enought to handle the 1080p movies, but then again that shouldnt be the case as i had no problem whatso ever whilst running xp os.

Allright, well uhm.. thank you for the effort of trying to sort this out for me. I really do appreciate it.
Hehe, im not planning on switching back to xp, windows7 makes a lot more sense, i like it very much.
Im going to get some better hardware, better cpu and video card to see if that solves this problem because im out of ideas, meanwhile im gonna have to stick with lower quality movies, because they work fine.

Well we gave it a go! thanks again guys, appreciate it.