Computer overwrites everything! Keyboard broken? Virus?

Since a short time, I have a terribly problem with my keyboard and my computer! That is why I am writing this text using my smartphone.

No matter what and how I am writing, my computer is always overwriting the upcoming characters or letters of the text I am writing in. For example, if I set the cursor between A and B of the text "ABCDE" and I am typing an X, the B is overwritten and instead of my desired result of "AXBCDE", I get the result "AXCDE". If I type another X, the text changes to "AXXDE" instead of normally inserting the characters.

Strangely, all other things are working as expected and like before. First, I thought it could be a virus, but perhaps, only my keyboard is broken. It is even not possible to log into my mailbox, because I cannot type my password in this way!

I am desperate, it does not stops! Did someone already encountered a similar problem with that insert over-writing thing and could find a solution? I am grateful for each help!