undefined key is pressed?

Hello, 

it feels like an undefined key is being pressed.

In some full-screen games when i type text, a character: "=" is automatically typed approximately once a second.

In some applications where I can create key-shortcuts "UNDEFINED" key is inserted.

In this forum, when a context menu is popped (with right mouse click) it disappears in a second because of the same reason. This is the most annoying thing, as I can't use some of menus.

I have tried this program to check my keys:
http://www.passmark.com/products/keytest.htm
However it does not show that any key is being pressed.

Any possibilities to fix it or somehow to omit that input?

edit:
I am on Dell Vostro 1000 and Windows XP OS. I have replaced my original keybord  (it went out of performance) with one like this: http://www.google.com/search?q=NC929