It is most likely that your new computer does not have the H.264 CODEC, where you previous one did. Install either the Apple H.264, or the MainConcept H.264 CODEC, and you'll likely to be working like before.
I have installed quicktime along with the k-lite codecs.
It is working as a gem right now.
However, I have another question.
I recently bought a new camera which records videos in AVCHD formats (.MTS)
Is there a codec that will allow import of MTS files into Adobe Premiere CS4?
CS4 includes native support for AVCHD. Nothing more should be needed.
What model camera?