It has happened to every one I know that has a printer. You start printing out something and you notice that there are horizontal lines running across the page. FREAK OUT!!
This simply means two things, either your printhead (print nozzles) are plugged up or dirty, or it means that your cartridge is running out.
Now for the latter, it is a simple fix, get a new cartridge or get it refilled. I like getting mine from http://www.oinkinkstore.com
What about the nozzles or printhead? Well this is a little more involved.
1. Run a cleaning cycle. The software that runs your printer (printer driver) does more than just tell your printer to print. It also can change paper size, quality, colors, and maintain your printer. Easiest way to find it is (in Windows, I do not have my Mac hooked up to a printer yet) by going Start>Control panel> Printer & Faxes>Right Click on your printer>Printing Preferences. From there find a button or tab that says Services/Maintain/Utilities. In there it will have a cleaning function. Now you might have to run it a couple times (up to 5 times).
2. Manual Head Cleaning. Now this can only be done on printers that have the printhead attached to the tank (HP, Lexmark, and Dell are the main three that do, however, there are about ten Canon’s that are like this). Get a warm damp paper towel (we use distilled water, but tap is fine). CAUTION You can Damage your printhead by doing this so be careful, and proceed at your own risk! Wipe the printhead slowly across the paper towel, using a little force to clean any gunk (dried ink) off. Then firmly press the printhead on the towel and hold it. If it is a color make sure you get all three colors, and black, well you get the idea. You still might have to run a cleaning cycle on the printer, but it should look better.
Now if you notice a ding or dent in the silver area on the bottom of the cartridge (The printhead), your cartridge is toast, and needs to be replaced.
If those tips do not help, take the cartridge to where you purchased it and see if they can help you with fixing the problem. Remember that regular use keeps the cartridge fresh. Think of them like an ink pen.