My Samsung ML-2010 USB printer was working well until I tried to share it with a Windows box.
After giving up on being able to share it, I reset all the options I had changed back to their defaults.
However, I can't print anymore.

Going into printers in control centre as ordinary user tells me that the print system currently being used is localhost:631.
However, when I change to Administrator and put in the password, it then tells me that the print system being used is: /var/run/cup/cups.sock

Then an error message appears:
Please can someone help me fix this?

sounds like KDE was compiled without cups support built in properly, i roll my own KDE and use the parameter of --sysconfdir=/etc and KDE's printer config tool works good...

you can also configure cups by going to http://localhost:631 in a web browser...

Thanks, I'll try that.
However, It's worked fine with KDE until now...

More info: Every time I send a job to the printer, it just sits there in the queue, marked processing.

oh. And one more thing. Every time I try to set the printer up again. I get through the driver setup and click 'test' to print a test page.
This causes an error: 'Unable to create temporary printer'

And then, after that: 'Unable to create printer. Error message received from manager: client-error-bad-request'