After installing .NET 3.0 on one of my test servers, I could no longer connect over RDP. From the client, the connection would just terminate with no errors. After looking at the Windows Event Log on the server, I found:
After looking around, this appears to be an issue with the installed nVidia driver, though ATI users have experienced this as well. Adding the following registry value and rebooting, I was able to RDP again.
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management] "SessionImageSize"=dword:00000020