Habe diese anleitung in einem englischen Forum gefunden. Bei mir hat es tatsächlich funktioniert! (nachdem ich vorher einige Nerven gelassen habe und der Lexmark Support mir im Prinzip gesagt hat, ich soll mir doch nen neuen Drucker kaufen...)

The solutions to install Lexmark c500 to USB (laptop or PC), NOT in LAN:

1. Download Vista x64 driver: Lexmark - Page not found
2. Right click -> Run as administrator
3. It will "install" the driver... and you must chose WHERE to install the driver (c:/Lexmark/Vista - I think this is the folder where it will be installed)
4. An Wizard appears (you can close it, it doesn`t matter, will not work!!!) AND THIS IS HOW YOU REALLY INSTALL:
5. click on Start->right click an Computer (My computer)-> Properties.
6. on the left you have: DEVICE MANAGER. Click on it
7. in that list you have something like: Unknown Device (click on it) and you will find LEXMARK c500
8. right click on LEXMARK c500 -> and click on UPDATE DRIVER SOFTWARE
9. chose "BROWSE MY COMPUTER FOR DRIVER SOFTWARE", and chose the instalation folder (c:/Lexmark/Vista)!!!!!!!!!!!
10. this is all! now it will work!