Based on the information that you have provided one could be here writing a long list of causes. However start with: Bad cable, wrong drivers for the OS.
If this doesn't help try a NEW question with LOTS more information about the problem.
T
August 2009
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix printers.