A week or so ago I see google changed something about that search box. It does behave a lot differently so my bet it's not your machine that needs fixing.
Try clicking on PREFERENCES and how do you have yours set?
" Query Suggestions
Provide query suggestions in the search box.
Do not provide query suggestions in the search box."
On the Google Homepage there is a box in the middle in which to enter your search. I used to be able to put my pointer on this box and it would turn into a straight line and former searches would show in a dropdown menu. This was very convenient as I often wanted to go back to a previous search and I didn't have to retype it. Now that function has disappeared from the Google search bar and I would appreciate it if someone could advise me how to reactivate it.