I had the exact same problem with a HP DesignJet 650C. I finally got it to rotate properly by re-installing a driver, but before I installed it I went through the following procedure.

I am assuming you are using Windows XP.

Click on "Start", then "Printer and Faxes". Click on "File" and then "Server Properties", then "Drivers". This will give you a list of all the printer drivers you have installed. Highlight and remove all the drivers that are listed for the printer you are having the problem with.
After you have removed all the drivers for the HP 1050C, then you can install the driver for the printer. In my case I had more than one driver loaded for my printer, and that created the confusion.

Good Luck,
Myladin