Here's a couple of thoughts:
1. Verify that the machine you are logging into has a password set for the user account (no password = no RDC!)
2. Verify that the login credentials you are using are accurate.
3. Turn off simple file sharing on the target PC (just google it, it's really simple).
4. Turn off the windows firewall on the target PC..even if there is an exception for remote desktop set, i've seen many cases where unless the firewall is disabled, you get no connectivity.
HTH
CCNA, CCDP, Net+, A+
Work Smarter, Not Harder....