Laptop "types" letter "Y" continuously as if the key were pressed, even without the keyboard

Have you tried to boot with the keyboard disconnected but a USB or PST keyboard attached?   Just to validate the keyboard is not throwing the character?

If it is the keyboard, you can probably find cheap parts or a parts only unit to use.

Have you scanned your system for viruses

or sticky key

could be a faulty keyboard controller, or motherboard going bad.

I hope this helps !

You said it is a laptop so how did you disconnect the keyboard actually? Did you physically disconnect the keyboard that is on the laptop or another one attached to it?

Do you have the 'built in keyboard' still attached? Or did you remove it?

If not 'try' to remove it (Check for manuals on the website) and use it with a USB keyboard.

test if it happens when you boot from a live cd like :
 ftp://ftp.kernel.org/pub/dist/knoppix/KNOPPIX_V5.0.1CD-2006-06-01-EN.iso           Knoppix      
www.nu2.nu/pebuilder/                               BartPe

or pu t in another disk, and do a fresh install to check
this will show if it is hardware or not !

Thanks for the ideas. I'm going to work on them and let you know how it goes.

ok - i'm waiting..

Ok, this is what I have done so far after the motherboard cleaning:

1.- Disconnected the built in keyboard and plugged in a miniDIN external one (remember is an old laptop). When I turned the laptop on this way, the problem seemed to be solved, but after I accidentally pressed the button that deactivates the touchpad, y's started to appear again.

2.- Inserted back the CD-ROM and booted from UBCD. The same thing happened. It worked well until I pressed again the touchpad deactivation button. And

3.- Ran memtest for about half an hour and didn't find any errors. I did this because sometimes when I try to restart the laptop after the problem arises, it won't boot at all. It freezes at the Compaq´s logo screen. Then I discovered that this freezing was caused by the y's themselves.

Now, regarding all your comments and questions.  Unfortunately an external keyboard is not a solution. A virus, sticky keys configuration or OS problem is out of question since it's happening using a live CD also. And it is not a keyboard problem because it is not even plugged in. Which leads me to think is definitely a motherboard problem, and specifically some sort of short circuit on the keyboard bus. My question now is how can I track it down? or should I just change the entire board?

Any help will be appreciated.

Now it's probaly come down to - is it really worth the effort? If you are earning let's say $30 per hour, yout time is valued at least $30 an hour. You already spent many hours with it and planning again. You have to buy this and that - is it really worth the effort instead of getting a new one?

You may be doing it for somebody else or like the "antique" stuff - then it's a probably a different story after all ;)

btw is there an option to update BIOS?

@ osmystatocny:   MEGA DITTOS

Thanks for your comments again, and nobus, that's exactly the info I was just looking for. I'll let you know if I could fix it, but definitely I'm giving you the points.

always interested in the outcome

With the information nobus provided I found that the keyboard controller on this machine (Presario 2150LA) is a National semiconductors 176pins PC87570 that controls not just the internal and external keyboards, but the pointing devices too (touchpad and/or an external PS2 mouse). So I double checked the surrounding areas to that chip for traces of something strange, cleaned the area again, and resoldered the chip using a heat gun but the problem is still there. This leads me to think that the problem is inside the chip as nobus suggested, so I'm leaving it that way since the only reliable solution would be to change the entire MoBo, or buying a new laptop.

tx for feedback !