1- Check that Windows firewall is turn off or permitting port 135
1a- check that Dcom is enabled - http://www.lansweeper.com/kb/accessdenied.aspx
2- in cmd run telnet ipaddressofmachine 135 (to make sure you can connect)
3- use the testconnection tool to test your admin/user credentials
This is due to the windows Firewall being activate on these machine.
ex like installing SP3 reactivates the firewall...
Groupe Rodeus