Distortion when moving Cubase from PC to Mac

I recently moved from a PC to a Mac and moved my copy of Cubase Studio 5 across. Works fine when recording new material.
I moved across the files for some of my older songs, recorded in Cubase 3 and it converts them and opens them up, but when you playback it sounds as if there is massive overloading on all of the tracks.

Any idea how I can sort this out?