Just this morning when I turned on my computer I got a Pri Slave HDD failing error at startup after RAM and drives initialization.

Weired thing was that it did recognized both HDD (togheter with the CD/DVD Roms), but got that error.

After restart windows XP loaded (the bar that is moving) then logo dissapeared and it remained on a black background not doing anything.

Third restart fixed this.

What all this mean?

First thing to do? Make a backup. Right now.

After that, check that the IDE connectors are clean and firmly plugged in. Also check that you've no strong beta-particle emitters aimed at the cable. That's been known to bugger things up.

Beyond that? If a hard drive's failing, there's not a right lot you can do to stop it. Run a full scandisk with a decent utility (please note the adjective) - see if it turns up any bad sectors.