From LedHed's Wiki
If you're trying to run psexec and are getting a Login Failure and you've tried all the simple troubleshooting steps like:
- Disable the Firewall
- Verify the username\password are correct
- The server and rpc service are running
- The administrative share is working
Then you may have to change the workstations Local Security Policy.
Start -> Run -> secpol.msc
Then navigate to:
Local Policies\Security Options\Network Access: Sharing and security model for local accounts
Change it from Guest to Classic
Reboot