In my experience I have had issues like this if not running eclipse as Administrator. Make sure that you aren't being blocked here.
EDIT: Please note Chris Stratton's concers regarding running an IDE as Administrator, as seen in the comments below.
EDIT 2: In my more recent experience, I have noticed that if I installed the SDK by copying and pasting the contents of the downloaded, compressed, folder to a destination rather than extracting it first, I would be required to run as Administrator. If this is the case, delete the SDK that you copied out of the compressed folder and simply extract the contents of the downloaded compressed folder. I am running Windows 7 so it is likely that this applies to all Windows 7 and 8 OS's.
In another case, I have also seen issues like this if the JRE is installed second rather than first. In this case, a reinstall of the SDK is probably all that is required.