I had the same problem with a few of my computers.
Turns out the problem was with the default security configuration on Windows 2003. To get it to work, I had to disable SMB packet and secure channel signing, by doing the following (per
http://www.digitalissues.co.uk/html/os/ms/smb2000.html):
http://www.digitalissues.co.uk/html/os/ms/smb2000.html wrote:To disable SMB packet and secure channel signing enforcement on Windows Server 2003–based domain controllers
1. From Administrative Tools open Domain Controller Security Policy
2. Smile
3. Select \Security Settings\Local Policies\Security Options folder.
4. In the details pane, double-click Microsoft network server: Digitally sign communications (always), and then click Disabled to prevent SMB packet signing
from being required.
5. Click OK.
5a. In the details pane, double-click Microsoft Network Server: Digitally Sign Communicates (If Client Agrees) and then click to "Disabled".
6. In the details pane, double-click Domain member: Digitally encrypt or sign secure channel data (always), and then click Disabled to prevent secure channel
signing from being required.
7. Click OK.
To apply the Group Policy change immediately, either restart the domain controller, or type gpupdate at a command line, and then press ENTER.
If you change these setting and still have a problem, try checking in the following area.
1. Open Active Directory Users and Computers, right click the Domain Controllers container and click Properties
2. Click the group policy tab and then on edit
3. Under Computer Configuration, go to the Windows Settings\Security Settings\Local Policies\Security Options folder.
4. In the details pane, double-click Microsoft network server: Digitally sign communications (always), and then click Disabled to prevent SMB packet signing
from being required.
5. Click OK.
6. In the details pane, double-click Domain member: Digitally encrypt or sign secure channel data (always), and then click Disabled to prevent secure channel
signing from being required.
7. Click OK.
To apply the Group Policy change immediately, either restart the domain controller, or type gpupdate at a command line, and then press ENTER.
I hope this helps!
TheEndless
[edit]added 5a for 2.0+ beta compatability[/edit]