stuttering crackling choppy sound and video playback

i have a dell i6400. with the sigmatel HD audio, and radeon x1400. recently i ran into this problem where any and all sound playback is stuttery, or crackling about every 3 seconds or so, and the video is extremely laggy as well. i can't remember exactly when it started, but i was getting the blue screen with it telling me memory dump and all that, so i googled the stop code and it said something about the hardrive being bad, so i ran the 32 bit diagnostics that came with the computer, and that determined that the hardrive was bad. i got on the phone with dell and had them ship me a new hardrive, and after installing that the problem wasn't gone, but i wasnt getting the blue screen anymore..

anyways, my roomates computer ( gateway w730-k8x ) just started doing the same exact thing with the sound and video being choppy, minus the blue screen. we never transfer files from one computer to another or anything like that, the only way they are connected is through the router, and we both have virus protection and firewalls enabled. i did all the virus scans, including a couple internet based ones, i.e. housecall. i did all the spyware scans with adware and spybot s&d. both finding nothing. i even did a low level format of the hardive and reinstalled everything. all drivers are up to date with no visible problems in the device manager. i think its only bad luck on both parts that we have the same problem, but if anyone thinks they are linked..

i've searched threads for duplicate problems, and found similar cases, but no real luck. If anyone has any input, it would be great to hear from you.

thanks,

M