right, first off i would like to say these are movies off my hdd, not on the internet.

when playing movies, and music. it every so often lags (best way to describe it tbh)
now because i have a scan warranty i asked them for help, after running a memtest and hdd test and reformatting my computer, the problem was still there, so i sent my computer back
a week later, and after them "watching e3 on my computer" to test it, they say there is nothing wrong (oh and they reformatted again?) and have sent it back
so i set it up at home, and put my movies back on, watch one and about 30mins in i get the same lagg

now this cant be a software problem, i didnt download anything and the movies work on another computer fine

so what could it be guys? i have little faith in scan technical team anymore (they also suggested i use internet explorer instead of chrome since IE has built in accelerators??)

By lagging, do you mean it gets jumpy and drops frames? What's your CPU utilisation when this happens? Is anything using processor time except the player? Is anything else accessing the disk?

What's the exact format of the files with the problem, what software are you playing them in, and what is the spec of the PC?

like it stutters and make a zzzz sound

spec of comp
intel i7 920, stock clock, 3gb ddr3 ram, nvidea 260, 500gb samsung hdd
nothing else is using the cpu, i had windows live and steam downloading things but thats all, in the past ive had nothing else running and tbh with my cpu i dont expect a couple of things like that to trounce it!
the movie is in .avi and was using windows media player (not downloaded vlc yet)

I mean, what format is the video actually encoded in. Frame rate, resolution, bit rate and codec. You should be able to see it in the properties of the file.

I've just noticed you say it does it playing music, as in no video just sound? If you're really not seeing any CPU usage at all from anything when this happens, then all I can think is either an interrupt issue, overheating or a dying hard drive.

You can check your hard drive and temperatures easily enough with something like scandisk and speedfan. Interrupts is less easy; do you have any other hardware plugged in? What's the sound? Is there anything in event viewer?

Do you have the same problems playing games?

Try playing them in VLC, it could be a Windows Media Player problem. Also, have you installed the DivX or Xvid codec? If they are AVI's there is a good chance they are encoded with either DivX or Xvid - AVI is just a container. An up to date codec could make the difference.