I'm typing in the middle of a text, everything is normal, the old text is pushed forward. Then suddenly, I think because I hit a special key on the keyboard, the old text is not pushed anymore but erased as I type. new letters replacing old ones.

I never found which key to press to reverse that.

It happens everywhere it seems. can be on this message board box for example or in notepad etc.

Please help!

It's the "Insert" key. It toggles that behaviour on or off. Above the "Delete" key on my keyboard.

Thanks!

Is there a way to disable that?

Not without some extensive modification. It's an inherent characteristic of edit controls.