Was the XP machine working OK when editing video, and then stopped?
Have you tried using QuickTime?
Do you have any other programmes open at the same time as the video editor? Ie: IE, Outlook/Word/Media Player...
How much memory do you have installed?
Have you looked at Task manager to see if something is hogging your CPU or memory? (Right click the task bar, Task Manager)
Try
K-Lite Codec Pack.
or,
Any Video Converter.