any1 know a player/codec for .mkv files?!?

You should really try SwiftPlayer(http://www.swiftplayer.ffmpegbox.com), It can play nearly(maybe exactly) any file format.

What I want to say is that the fastest and least resource-demanding video player is none of those listed above. The champion is SwiftPlayer(http://www.swiftplayer.ffmpegbox.com), you should really try it. It is by far the fastest and least resource-demanding one and can play nearly(maybe exactly) any file format.
I try to play some HD 1080P high resolution videos using my laptop(1.60 GHz CPU, 512 MB Memory, Windows XP), no video player other than SwiftPlayer can do the job!