FCP studio, QuickTime strangely not working (long story)

So, a brief setup:

First, the G4 and G5 at my work wouldn't start up Mac OS (10.4.11). This happened at the same time. I reset the PRAM on each, and the G4 worked fine. The G5 still wouldn't start. I started it in safe mode, removed all login items, and restarted.

Now it appears to work fine: Safari, VLC player, iTunes, all OK. But anything Final Cut or QuickTime Player is not ok. MP3's will play in iTunes, but not QuickTime Player (7.5.5), .mov files will open in VLC, but not in QuickTime. I get the error message "This movie could not be opened. Mac OS error -39)

Also, Final Cut Pro (5.1.4) and other FCS programs are not working. In FCP, instead of showing the video of a project, the viewer and canvas windows are just green. DVD Studio Pro (4.1.2) cannot find its assets. These are for projects I know are properly connected.

Any thoughts? I'd surely appreciate it, I'm a bit of a newbie; apologies if there is an obvious solution.

Thanks!