Too slow and don't boot anymore

Hello,

First of all, sorry for my bad english ...

I have a 10 months old PowerBook 15" 1.67 Ghz, 1 Go RAM and OS X 10.4.6.
Two days ago my computer became very very slow. I could move the mouse but that's all and suddenly my session logged off. I wasn't able to log in again : OS X said something like my session is not ready to use (?).

I forced the restart but i had nothing more than the grey screen with the apple logo (very first step of the boot process).

Yesterday i decided to re-install OS X 10.4.6. The (clean) install was fine, nothing special to say. After about 1 hour of restoring my data (with Backup 3) the computer becomes very slow again...

Second clean install last night. The computer works all the night for restoring (again) my data and this morning it was again very slow (I could see the Backup 3 window saying all the data has been recovered). Again, it automatically logged off (while unmonting my external hard drive). I could restart cleany the computer but the boot process stops again on the grey screen...

Any idea about this problem ?

Thx a lot