Problems at start-up, runs slowly, then freezes.

Hi.

My 2 and a half years old Mac Pro 8 cores was running quite slowly and re-booted randomly, so I decided to reset the PMU or whatever it´s called by pressing that little grey button on the mainboard.

For a week or two, everything was fine, but today suddenly I just couldn´t start it with my user account: after entering my password, a blue screen appeared and I heard the disks working, but then returned to the username/password screen. I tried to use the guest user account and other account with administrator privileges, and they worked for a few minutes, although the machine was slow and finally it froze. I could move the cursor around the screen, but nothing else.

I managed to check the start-up disk with the disk utility , and it said it was OK. I tried to start-up in safe mode and it didn´t help. What else can I do? Run the hardware test with the OS disc?

Fresh OS reinstall?----> And if I do a fresh reintall of the OS, am I going to keep everything I have in a RAID 0 set of disks?

Thanks a lot for your answers.