My computer has positively refused to play video properly for some time. Like yours, pictures get very pixellated and the picture itself is very jerky. The results are consistent across Windows Media Player, Quick Time, VLC Media Player and others.
A few days ago I disconnected my computer from the internet and shut down Zone Alarm Internet Security. The few short movie clips I then played were perfect but don't get too excited. Following a personal recommendation I have now uninstalled Zone Alarm and installed Kaspersky. However, as soon as Kaspersky was running the video problem returned but nowhere near as bad. As I was told that Kaspersky was far less resource hungry than ZA I came round to the conclusion that my problem was one of resources.
Until today my computer was running XP Home SP3 with a 3.4 Ghz Pentium 4 and 2Gb of RAM. My graphics card is an ASus EAH4350. My screen is an LG 19" widescreen and is set to 1440 x 900 with a 60 Hz refresh rate (I have also tried 75 but it doesn't seem to make much difference).
I have now upgraded the RAM to 4Mb DDR2-1066 (I know that XP won't utilise all of this but I am thinking of upgrading to Windows 7 64-bit). The videos now play almost flawlessly but still suffer from the occasional slight jerk. Incidentally, I put a few of these short clips on to a DVD using Serif Movie Plus X3 and they were absolutely fine so the problem is not with the video.
Sorry if this is a bit of a disjointed ramble but it has been my experience so far.
Edited by hamluis, 19 April 2011 - 04:52 PM.