Moving to the MacBook might be part of the issue. The older PowerPC only components wont work in the new Intel Macs. It might be wise to remove all the old 3rd party components you moved or installed on your MacBook and replace them with freshly downloaded universal binary versions. That can also be tricky, as not all the component authors have universal binary versions.
For the case of Divx, you can grab their universal binaries here:
http://www.divx.com/divx/mac/
And the latest universal binary of the AC3 component can be found here:
http://web.mac.com/kaicherry/iWeb/TheCabaal/Dark%20Ten/8F6C946F-54D4-45D7-8BDD-0D3B2D8B385C.html
We are always looking for more universal binary components to extend QuickTime's range of media playing ability so feel free to post them when you find them.