So I was wondering if I could change the default player in the preview window to Kmplayer?
If not is there another way to have the preview be not as choppy and laggy?
On a laptop with 2GB of RAM and a "bad" CPU? No, not likely. The best course of action is to get a new computer; I'm not trying to be smart-alecky, but your system is just probably just not up to editing HD. And editing HD is a lot different than playing HD, so it doesn't really matter what you can do with the footage outside of Premiere Pro.
>default player in the preview window
As far as I know, what you see inside PPro IS PPro, not some other player
Laptop Video Editing PC http://www.sagernotebook.com/
-or another brand laptop with the following features
-the newer sandy bridge 2720 or 2820 quad processor
-and nvidia graphics preferably the 460m, 485m is a bit much
-1280x900 display with OpenGL 2.0-compatible graphics card
-and 8 or 16 gig ram and Win7 64bit Pro
-and 2 internal 7200 HDDs minimum
Default Settings is the Adobe Player.
If you run Cineform e.g. you can set it the CFrtPlayback.
Unfortunately not with the KMplayer.
But as Colin already stated. Looks like an underpowered laptop.