Keyboard problem, ALT, Backspace, A

Hello everybody!
This is the first time I have used this forum and I hope I am not asking anything stupid. I have got a problem with my keyboard and I can't help it or find an answer on the internet. I am using a Mac OS X, Version 10.5.8.
The problem looks as if the Alt key had suddenly got linked either to the "A key" or to backspace.
I'll explain better.
If I press either the A key or the left Alt key, the result is that I always get what I'd get if I pressed the two keys together, that is to say å with the italian keyboard and ¯ with the finnish keyboard. I suppose that if I used the English keyboard I would get å as well.
On the other hand if I press either the right Alt key or backspace, I get what I would get if I pressed these two keys at the same time, that is to say I delete the whole last sentence and not only the last letter.
This is quite bad most of all because I can't write the A key anymore. I am writing this message with a USB keyboard: I have tried two USB keyboards and the all work just fine.
It seems to be a problem of the laptop but then again I think it might be some weird setting that I activated. In fact I started to have this problem while I was writing and I think I might have pressed some wrong key combination.
I really hope somebody could help me, thank you a lot!

Sounds like you've you've got a stuck Option key.

If it doesn't clear then its off to the Apple Service Centre in your area.

Yeah, it is strange though because the A key and Backspace give problems, all the other keys work as usual. If there was a stuck Option key i suppose it would always give problems. Furthermore if I press one of the Alt keys with another key, they work as they should.
In few words the problem seems to only about A and Backspace.. and about Alt, if it is pressed alone.

Hi, while trying to solve this I discovered something.
I deactivated the Alt key of my Internal Keyboard (System Preferences > Hardware > Keyboard and Mouse > Keyboard > Modifier Keys > Option Key --> I switched it on "No action").
Now it happens something weird because finally I can write my A but I can write it both pressing the A key and Alt (left). The same happens if I press Backspace and Alt (right): in both cases I delete the last letter.
Any idea of what could be wrong with my internal keyboard?

Pardon, a little mistake: the outputs of Backspace and Alt (right) haven't change differently from A key and Alt (left)...

a couple of other things to check.

Disable your fix.
In System Preferences > Language & Text > Input Sources
check the country settings are correct.

Also enable Keyboard and Character Viewer, at the top of the list.
This will put a Menu up by the Sound icon in the Menubar.
From that select Show Keyboard Viewer

This will show you what Keys are pressed.
Press the Shift, Option and you'll see how it works.

If the option key is having problems it should show here.

Nope..
I had already tried to show the Keyboard Viewer. But it is strange because according to the viewer there is no problem, but the computer writes different things that it shows with the viewer...

Sounds like a hardware problem, you'll need to take it to your local Apple Service Centre.