Microsoft only really has 1 character: Master Chief. Not many other choices for them
The paper clip office assitant from Microsoft Office is memorable, unique, and even pre-dates master chief. Either him or the Windows Logo.
Cheese String man would also be a great contendor from Lactalis (Sorrento). Or Chester Cheetah from Frito-Lay. Although if Pepsi needed a rep, I'm not sure if they'd go with Brittany Spears (pre/post pregnancy alternate costume) or Micheal Jackson (black/white skin alternate costume).