Problem
You want to be able to minimise all the open windows to get to the desktop. There was a button in XP but it's gone in Windows 7
Solution
It's still there, but now you need to right click on the taskbar and select it from a menu. Just right click on the taskbar and select "Show Desktop"
A nice new feature in Windows 7 is that when you've finished you can right click on the taskbar again and select "Show open Windows" to go back
If you just want to keep one window open and minimise all the others, there's a new feature in Windows 7 called "aero shake" that allows you to do this. Just click the title bar of the window you want to keep open and drag (or shake) the window back and forth quickly, and the other open windows are minimized.