You need to run the program as administrator (please don't think you are doing this already; you aren't). Use the link to the FAQ in my signature, and read the post 'getting to know Eraser 6'; the relevant instructions are there.
It is better to do this from a user account with admin privileges. Trying to use the administrator account brings complications of its own.
Thanks I've done that; but what is the difference between a user account with admin privileges and an administrator account ?
I use Windows 7 and if I go into the accounts to change them I am given the choice of either being a standard user or an administrator.
User accounts in Windows 7 either have or do not have administrative rights, according to the way they were set up (as you say). Typically, the main (or often only) user account is set up with admin rights.
However (and this is the important bit, and is different from what happened in Windows XP), programs are nor run with admin rights by default, even when the user has admin rights. The program has either to be run explicitly as administrator or its properties have to be changed so that it is always run administratively (aka 'elevated'). Many utilities (e.g. CCleaner) run in this latter way, but this does not work for Eraser, as for some of its functions (e.g. drag and drop erasing) it actually needs to run normally.
You will have discovered that the procedure to run Eraser administratively is complicated by the fact that the running process which starts by default with Windows does not run administratively, and has to be stopped before the program is run elevated. This is the procedure described in the FAQ. Joel is working towards replacing the running process with a service in Eraser 6.2, and with any luck this will mean that free space erasing will just work, as file erasing just works now.
Perhaps wrongly, I took your first post to mean that you were running Eraser from the (normally hidden) Windows 7 Administrator account. As I said, this is not really advisable; the Administrator account is provided for rather limited and specific purposes, and most users do not need to use it (or even know that it exists).