Until today I have never encountered problems taking over my old Windows-machine. But now it doesn't work anymore... Could it be that something changed with an update of my Ubuntu (10.04) which caused this?

The Windows machine does not update or change, and I cannot imagine that I changed something myself at that machine...

I can still reach it using "smb://192.168.1.33/"

But when I try Remotedesktop Client with that IP it gives the message;
Error (grdesktop)
Autoselected keyboard map en-us
ERROR: 192.168.1.33: unable to connect

When I try using Terminal Server Client (I read in another thread that could solve the problem) I get the same response...

Where can I find out what the problem is?

Make sure port 3389 is open on Windows. Also, check Event Viewer for more details.