Christo:
Do you want to build a serial or USB based joystick? Serial would be relatively easy to build, you just need to add something like a MAX232 or equivalent chip to translate from the Atmega's 5V to the required +- 12V as required for serial communication. This is probably a dead end though since serial ports are quite scarce on modern computers.You could build a USB based mouse by implementing the HID interface for a mouse using the V_USB library (http://www.obdev.at/products/vusb/index.html). An Arduino port of this library can be found at Google Code Archive - Long-term storage for Google Code Project Hosting.. This will be a little bit more tricky and will require some internet searching for examples to adapt. Some examples can be found at http://www.obdev.at/products/vusb/prjhid.html but are not specifically adapted to the Arduino environment.
That's what I was searching for. I wanted to make a USB based joystick. I took a look at the links and it sound extremely complicated. I'll try to read more about USB connection, maybe that way I can understand a bit more from, how I should adapt and configure it the right way...