playing videos in vista results in choppy audio/video

This is only a problem I have in Vista. I have Vista Ultimate.

What happens is that every time I play a video I get this occassional
stutter. These are not shows I'm recording or anything like that from Vista;
these are music videos that are downloaded. I know the problem isn't the
video itself b/c the stutter does not happen in the same spot all the time.

The steps I have taken so far to fix the problem are:
1. I noticed the OS came with a driver for my video card. I found the
driver specific from nVidia and installed it. I have a Leadtek PX6600 GT-TDH
but it's just a modified nVidia card.
2. The OS comes with a default plug-n-play monitor driver. I have a Dell
P793 monitor so I downloaded the most recent drivers from the Dell website.
3. I verified that DMA is turned on for all hard drives.
4. I checked Event Viewer for any hardware failure messages... nothing.
5. Verified disk defragmentation is taking place. Currently scheduled to
run every Wednesday at 1am. I always leave my computer on so it did run.
6. I verified these videos all play just fine in Windows XP.

My take on the situation is that it's a driver problem. Not sure how else to
diagnose and fix the problem at this point.

It could just be a codec problem...
I had the same problem here using media player...
but I downloaded k-lite codec pack and then vlc videolan player...
and everything worked fine here...

I did a little more research. What I did find is that the choppy playback
only happens when the video file is on my SATA drive. All my other drives are
standard IDE drives.

I lied. I have two standard IDE drives just for storing non-OS specific data.
I moved the video file from one to the other and the playback was fine. Any
ideas why one of the drives would be causing the choppy playback?

I have been having the same problem with Vista Business on Asus U3Sg laptop (2.5GHz core2duo, 3GB RAM, 250GB HDD, nVidia GeForce 9300M G).

Have checked many of same things (DMA/defrag/registry error scan and defrag/malware/virus/trojan scans) and no joy. I have even set Vista to lowest graphics setting...

The thing is the same files used to play back fine...
I have k-lite mega codec pack installed and vlc (which is the app I use to play all media files by default).
Vlc used to work fine as did windows media player (with the codecs' help)...
The files work fine played on another PC or on my DiVX/XviD capable DVD player...

The graphics drivers are the ones from Asus - should I get the nVidia ones?

Any suggestions would be great...
Too much longer and I will have to revert to my earlier Arcronis disk image of the system... -hope I won't have to.. as that was a bit too long ago now!

This is going to sound crazy. I was having the same problem. i went into
"manage add-ons" off the tools menu, and disabled all yahoo
add-ons...restarted IE, all problems solved. Go figure...