Keyboard issue after lock computer

Suddenly I have an strange issue with my keyboard (Microsoft Wireless
Multimedia Keyboard) on Windows Vista Home Premium x64. Problem
appears when I lock my computer ( [WINDOWS]+ [L] ) and then come back
to my session. Keys don't work in a normal way. Some letter keys don't
print its letter. Instead of this, strange things happen like open de
Run command window or My Computer or toggle within windows...

I discovered that in the lock screen, I have to clic on the
Accesibility button. A window with accesibility options appears and no
one is checked. I pressend on Cancel and then enter my session.
Suddenly, keyboard works fine again.

Why is this happening suddenly? It never happened before.

Thanks!