It is very likely that your new computer does not have a CODEC, that the old one did. In the case of H.264, here are three popular ones: Apple, Lead and MainConcept. With later versions of Apple's QT Player, their H.264 should be installed. Many users have found that they get better results from either Lead, or MainConcept, but this is usually on Export.
If you have a recent version of QT Player, I would uninstall it, and then re-install it, as it could well be that the H.264 CODEC is not properly installed. Or, one could download and install the Lead, or the MainConcept version.
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Also, what kind of camera or camcorder did this video come from?
It could well be that the codec isn't supported in Premiere Elements if this video is from, say, a still camera or a Flip-stylel pocket cam.
You are a top man!
I reloaded QT as you suggested and it worked. I would never have thought of that.
Many thanks indeed.
Great news. There have been a couple of reports here, and on the PrPro forum, where it seems that the QT H.264 CODEC gets smurfed (a very technical video-editing term, meaning "messed up... "), so maybe some OS updates, or similar are afoot here?
Glad that you got it sorted, and reported your success. That probably saved buying the Lead, or MainConcept, but keep those in the back of your mind, if you happen to have quality issues with Export/Share. That is where others have gone looking for a different version.
Good luck and happy editing,