I also think that Pierogi might be a good choice, but then I'm biased.

Pierogi can indeed be used as a remote control for Windows PCs, but only via an infrared port on the PC. (Most "Media Center Edition" PCs now come with IR ports. You can also purchase an IR receiver for your PC if you don't already have one.)

There are other options available if you'd prefer to connect to your PC by bluetooth or wifi. And I think there are better options than Rdesktop on the N900, if you're looking to run a full remote desktop.