I have a standalone Windows 7 Pro x64 machine which I cannot scan. Lansweeper says the provided credentials do not have administrative rights. This is not correct, I double checked. I tried two different accounts (both local admins) to no avail.
Make sure you run the script listed in http://www.lansweeper.com/kb/3/WMI-Access-is-denied.html It makes some necessary changes to allow scanning of workgroup computers.