'Needs elevation' is Microsoft-speak for running the program as administrator (which doesn't happen by default in Vista/Win7 even if your user account has administrative privileges). Eraser uses .NET 3.5, and will call the Microsoft downloader/installer for it if it is not detected on your machine.
Why the Microsoft installer chose to be difficult (unless the account you were using did not have admin. privileges) is a mystery. Assuming that it is an admin. account,I suggest that you download a separate .NET 3.5 installer from Microsoft, and right-click on the executable file to run it as administrator. Then install Eraser, which with .NET 3.5 already in place, will bypass that part of its installation.