Not memory - Almost definitely - And I say almost because you never know.
Normally I would say reinstall your video codec that is affected, but it seems to affect multiple codecs - Meaning it is probably a more generic, system-wide problem.
My best guess would be you are having a problem with DirectX - Possibly DirectX 9? Windows XP comes with DirectX 8.1 by default, but if you have upgraded to 9.0, this could be another bug. Problems with DirectDraw acceleration can make an identical problem from what you've mentioned.
Even if you do not have 9.0, it could still be a problem with DirectX... DirectDraw more specifically. Check your DirectX acceleration settings in Start/Run/dxdiag.exe. Click on the "Display" tab and look for anyting suspicious, like errors at the bottom or anything disabled.
You may want to try disabling DirectDraw and restarting. Then enable it and restart again... DirectX has been known to be stubborn.
You can always reinstall DirectX using a 3rd party program. Search on google for one - Just make sure it is for Windows XP and the specific version you have - Otherwise you might have problems.