I discovered that AltH doesn't work and I was unable to unhide objects. After a series of tests and looking online, I discovered myself that the left Alt key doesn't work always, but the right Alt (Alt Gr) does.
In User Preferences, if I try to edit certain commands that had already Altsome key, I press the left Alt and it appears in the box waiting for the other key, but if I press the key it doesn't catch it. But if I try with Alt Gr and then I press the key, it works.
For sure it does not take AltH.
Blender 2.78 and 2.79, Windows 7 x64, US International Keyboard.
It's not an hardware issue because I also tried with the on-screen keyboard.