Well that is not the case any more with Windows Vista and Windows 7. By default, it is disabled:
However, they do allow you to enable it - temporarily, if you want to use it.
All you have to do is press the Alt key when you have a folder open, and press the Alt key again to close it (or as in Windows 7 just leave it alone, it will automatically disappear when you close or switch from one open window to the next).
But what if you want the menu bar permanently on top when viewing your folders? Well, that is possible also:
First, open one of your Windows folders. I opened the Documents folder.
To access it, go to your Start button and left-click. The Start menu will pop up; go up to the very top of the right panel and left-click on the Documents folder (or whichever one you chose).
Your Documents folder will pop up and the top-left of the folder will look something like this:
To enable the menu bar permanently (or until you want to change it back), left-click on Organize. On the drop-down menu, left-click on Layout. A new menu will pop up:
Left-click on the Menu bar to put a check mark beside it. And voila! That's it!
Now every time you open a folder in Windows, the menu will be there. And to remove it, you just follow the same steps to un-check the menu bar and you're done!