Shutdown and User switch issues on a previously working setup

[mimizone]

I had 10.5 running for a year or so on a Asus P5W DH with a Quad Core cpu, Nvidia 7200 and Sata disks.

 

recently, the machine started to behave differently, being slow when disk was being accessed apparently. It was making osx or the application not responsive at all for several minutes.

I think one of my disk was dying (as it also behaved weird at boot time in the bios sequence) so I replaced it and reinstalled from a Time machine backup.

 

Now the machine seems to not suffer the slowness issue but I can't shutdown (never) or most of the time not switch user.

if I try to do so, it logs me out, and just show an empty blue screen with the cursor and the disk activity led is blinking, forever and ever and ever....

 

I am wondering if the disk controller is the real issue here or if it's just since I tried to install 10.5.6 twice unsuccessfully that I screwed up the install.

 

a detail that might be important, I am now booting using the OSX DVD as I haven't rebuilt the boot sector since I tried the upgrade to 10.5.6.

 

Any help is very welcome as I can't think about any thing to verify or look for.

 

thanks.