If this is McAfee's RootkitRemover tool, you may have found a program bug. Unfortunately it's not a mainstream product so it isn't supported. This is taken from "How to use RootkitRemover" -
RootkitRemover is being provided as a free tool to detect and clean specific rootkit families. It is not a supported tool. McAfee Labs makes no guarantees about this tool.
You've got Administrator privileges. Did you download it to the desktop? If you've got Vista or Windows7 you may need to right-click and select 'Run as Administrator'. If you still can't get it to run, I should leave it and try something else. If you're absolutely certain you've got a rootkit infection you could try TDSSKiller; otherwise, download and run Stinger.