After I upgraded to Windows Server 2008, I was unable to remotely log in via Remote Desktop. The client machine I'm trying to connect from is Windows XP SP2. When I tried from a Vista machine, everything worked fine.

Here's how I solved the "username or password is incorrect" problem:

First off, I noticed that the Routing and Remote Access service wasn't started on the server. To resolve this, do the following:

1. Click Start, point to Programs, point to Administrative Tools, and then click Services.
2. Right-click the Routing and Remote Access service, and then click Start.

The next thing that needed to be dealt with is the Remote Desktop authentication on the server. By default, Network Level Authentication was selected. Windows XP doesn't have the ability to authenticate using NLA. To allow our Windows XP client to connect to the server, you have to change this setting:

1. In Server Manager, click Configure Remote Desktop.
2. Choose Allow connections from computers using any version of Remote Desktop (less secure).
3. Click OK.

This still didn't solve the problem for me using my Windows XP SP2 client. When using XP SP2, you need to specify the domain name and the username (i.e. domain/username), not the server name and username. After I specified the domain with the username, I was able to successfully connect to the Windows Server 2008.