Windows 7 provides new feature called search history. It means that when you are typing in the search box previous three searches appears. In case if you need to disable this feature just follow the instructions below:

1. Go to "Start" and type into the search box "gpedit.msc" (without qoutes) and press enter.

2. Locate the following folder User Configuration,  Administrative Templates,  Windows Components,  Windows Explorer and click on "Turn off display of recent search entries in the Windows Explorer search box", then just choose "Enabled" option and click OK.

Now if you type something into the search box previous history will not shown. If you want to change back settings just follow the above instructions and choose "disabled" in Group Policy editor menu.