Fullscreen lagging

That sounds like the system is having hard time to process large amount of data (decoding).
Try the offline content with different players that may try to use the GPU power (as mentioned above).
Give a try especially with MPC-HC: Media Player Classic - Home Cinema - Video Player
or Splash: Splash - Next Generation Player

Thanks - using a player that is more efficient at decoding helped... a little. I can now watch video, but it's still not perfect.

So you are correct: the problem is the system being unable to process the decoding of the video, especially when having to blow up the picture to fullscreen.

I am very surprised at this though, because the T410 should be very powerful and actually, using the intel HD graphics, claims to greatly accelerate video of this kind.

So while the above work-around is a temporary solution, it's not a fix of the underlying problem.

Having looked around on the web it seems that there are other people with the intel HD graphics chipset who are having this problem. But nobody seems to have a real solution yet...

2 other things matter: the type of video (is it HD or Full HD, and the compression method used) and the type of Intel HD graphics chip set. Some of them, like Intel GMA 950 or GMA 3150cannot handle the modern HD videos, while others like GMA 4500MHD can handle heavy videos pretty well.

It's really any kind of video. Anything on YouTube, anything on similar sites, Flash, DivX, ... you name it.

I'm not sure what intel HD graphics chipset I've got.
My laptop is the standard Lenovo T410 (I don't have the optional Nvidia card... just the intel chipset):

Lenovo - Laptop computers - ThinkPad T Series - Technical Specifications

I'm just surprised that not more people are complaining about this. Maybe some work and some don't?

Run this tool to see what kind of graphics you have: GPU-Z Video card GPU Information Utility

This is what the tool says (attached screenshot)

[IMG]file:///C:/DOCUME%7E1/874073/LOCALS%7E1/Temp/moz-screenshot.png[/IMG][IMG]file:///C:/DOCUME%7E1/874073/LOCALS%7E1/Temp/moz-screenshot-1.png[/IMG]

Looks like T410 is using the integrated graphics chip that is combined in the Arrandale processor.
The good news is that this chip fully supports HW acceleration, so the solution is to try to reinstall the appropriate drivers.
Try first Lenovo support site.

captcaptain said:

I've tried many different players (divx, VLC, GOM Player, Media Player Classic, etc), but for every player, when ever I view a video in full screen, the video lags. The videos alternate between playing in really choppy images for a while to the video playing extremely fast for a couple of seconds. The audio is completely fine and streaming videos online/watching YouTube is no problem for me.

While some people believe their video issues can be solved by switching compatibility modes or that the lagging has to do with the player going into invisible mode (whatever it's called), I've tried the compatibility modes and disabling the invisible feature and the videos still lagged.

I got a new Lenovo T410 for college and my specs are: Windows 7 Professional x64, Intel(R) Core(TM) i5 CPU M 520 @ 2.40GHz , 4.00 GB Memory, Intel(R) Graphics Media Accelerator HD for my graphics card.

Any ideas?

I have "K-Lite Codec Pack" installed...
Try this:
With "Media Player Classic" open, go to OPTIONS > REPRODUCTION > OUT
Video DirectShow -> Choice the first one => SYSTEM DEFAULT > Apply > OK
Close and reopen the "Media Player Classic"...
Done.