I have some HD videos (720p) on slackware 13 but they're not playing as they should be no matter the player i am using - vlc, mplayer.
The sound is ok but the video is delaying and slow and it stops on like each 2-3 secs..Also when it plays it's like in slow motion so the video is "after" the sound...
What is the problem and how to fix it ?

im not a linux expert but what system specs do you have? cpu vga ram etc
i've played 720p movies on older machines and they were not capable

I want to say that i get this when i just go mplayer hd_video.wmv:

************************************************
**** Your system is too SLOW to play this! ****
************************************************

Hmm...What kind of hardware is required so that it would play nice (not in slow motion and stopping on each 3 secs ) ?
My current CPU is 2.4 GHz and my gfx card is GeForce 5200 FX 128 MB and RAM 768 MB DDR1...

you video card is the problem.. but you can play normal xvid or divx movies without any issue

the cpu is fine..
few years ago i had a Athlon64 3000+ which was 2.0ghz and was capable of 720p only.. 1080p was no go !

Okay so what is the minimum for video card in order to play 720p ? And what about 1080p - what cpu and what video card ?

from my knowledge 1Gig of ram, 2.0+ghz cpu 7600GT video card because i was using this model on my old rig for a 720p

for 1080p requirements click HERE

As a current hack you can try resyncing the audio/video in mplayer once in a while when it gets out of sync. There are several options to mplayer to attempt to do this automatically (-framedrop, or -autosync 0, for example) or you can press + or - on the keypad to manually sync forwards/backwards. What happens if you scale the video?

Interesting, I have almost exact same system specs, yet I recently played such video without a problem. CPU usage was about 80%, though.

Sorry...But your wrong.
The vodeo card is not really the problem. i have a sandbox that is a p4 1.7 fx5200 512mb ram that plays 720p fine ( make sure your using the correct driver) In the very start it was giving me some problems but after getting the driver right all was well. however, something to consider is that you should be thinking BIT RATE vs resolution when it comes to video.
are you able to run compiz? i am wondering cause if you can then you are running the card with a good driver for 3d support ( which oddly effects video playback )?

once you start getting into higher bitrates you will see a decrease in what it can run. you typical mp4 @720x480 might run on your machine but then you might get another file thats 720x480 .h264 codec which bogs the box. the codec also imposes an overhead to put the picture back together.

Are you using the nvidia drivers ? And along with it the xv or gl driver in mplayer ?

If so, then you should try converting it to x264, aac, mkv. wmv are not well optimized and will use lots of CPU usage. If you need help on this say so.