printer software installs drivers, those drivers are registered.
first, is there an installation directory that the partial install left (look in program files/the name of the company/printer).
If there is, try and delete it. If it tells you you cannot, reboot and then try again. if it still tells you you cannot, then we need to do a little hacking.
as for the registry, a simple way to find all of the associated registry keys is to do a find on the name of the program, and delete every key therein. might be tricky with printer software. take a look, be cautious, but not too cautious, it is not as easy to fuck up your pc as the techies will tell you.
Also, lets try and find the files it installed. go to your "search" tab and look for files that have been written to your hard drive very recently that have a ".dll" extension. see if they look like they would be from the printer software. if they do, we can unregister those dlls and delete them manually, and then try to reinstall.