Blue screen: my G5 fails to boot normally, only in safe mode

Question

Greetings,
After a power failure my iMac G5 started to behave quite odd.
That is, when I saw the computer was off, I pressed the power button, and it turned the coolers on and showed a screen filled with colored "snowflakes" (like one an untuned TV set shows, but static).
Curious, I forced it to restart, which ended in my current problem: +it boots, shows me a blue screen with mouse arrow in topleft corner, and this is it.+
Interesting is that when I move the mouse the cursor doubles: one arrow stays in its original place, and another one moves around the screen. Five minutes (or so) later the second arrow turns into a spinning beachball. After the blue screen appears there is almost no disk activity to be heard.
My next move was to try and boot from the Leopard DVD, which took much longer as it should, but ended in exactly the same way.
After that I tried safe mode, and here at last it worked. TechTool Pro's full disk check (including surface test) did not show any problems.
Reading the logs does not provide any clues to the problem. Everything is pretty much the same; I suppose it's just I don't know which log and where to read.
(By the way, I don't have the Application Enhancer which can cause a similar problem)
Any ideas on what's going on and how to deal with it will be very appreciated.
Thanks in advance!

Answer 1

There may not be a problem with your disk, but there certainly could be some problems with your file system. If you have or can borrow a copy of Disk Warrior, I'd use that to rebuild your directory with a new one; it will probably show a lot of problems in the report.
If you can't do that, then at least if you're in Safe Boot mode, the first thing I'd do is Repair Permissions, and then Verify Disk. If the Verify Disk report shows serious problems, you'll need to repair those before you can get a normal working system again. You should also try rebooting with the 10.5 Installer DVD again to see if it will let you boot, after you Repair Permissions and Verify Disk; if so, you can use Disk Utility from the DVD to repair your disk.