When I set up the Android Development Toolkit in Eclipse running on Windows 7, I ran into an issue installing the platform packages. Luckily I knew why it didn't work but anyone new to the stricter Windows 7 security settings may be at a loss.
So, if you run into issues installing the platform packages it is because you are probably installing them to a folder with higher security settings and therefore need to run Eclipse as an administrator for this to work.
One way to do this permanently is to go to the properties of the eclipse shortcut you are running and in the compatibility tab check the box to always run as administrator. Unfortunately you will always be prompted by windows when starting up. (I believe there is a way to turn this off as well)
NOTE: If your Workspace folder is located in your user folder, you will have to set everything back up under the administrator user.