HP Printers: 2600 and 3100N are printing streaks of lines.

The glass is smudged or dirty or the scanner bar may have dirt on it. Cheaper to just replace the machine.

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Your printer head - the ink cartridge in case of most HP deskjet printes, in most cases is defective and you need to replace it to have a clean print.
Anyway, read this tutorial, has some good tips:

HP Deskjet D2500 Printer Series Horizontal Streaks or Lines in the Printed...

For Epson printers, read this


Epson Stylus Color Ink Jet Printer


Epson Printer Horizontal lines


Same for Cannon priters


Horizontal lines when printing with Canon ip4500 Forum Thread

Drum or photoconductor will cause streaks/lines

Vertical streaks:
Check,which color/s streaks are appearing on print out,remove that cartridge/s check on drum surface for streaks.. if found replace that cartridge/s..check.
Horizontal streaks:
This problem appears when the cartridge/s image drum not rotating properly. Resit the cartridges properly, print 20nos of blank pages, then check.... replace the cartridge..

Depends on whether vertical or horizontal streaks. Go here for guidelines on fixing issues.


http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=c02206095&prodTypeId=18972&prodSeriesId=4066450

Fill an old black cartridge with hot water and scan and copy something with a lot of black on it. cleaned out my black ink clog after about 12 copies. Put new ink in and it's working fine. Oh yeah, use a syring minus needle to fill lower o-ring on back of cartridge.

If your printouts are streaked or faded, missing black or other colors, have blurred or fuzzy text, or ink streaks or smears, or show other problems, please refer to the link below for troubleshooting steps for print quality issues.
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01957673&tmp_task=solveCategory&lc=en&dlc=en&cc=us&lang=en&product=2512009

adjust your thickness guage as card stock is 3 or 4 times thicker than a 20lb plain paper stock. Here is link to a helpful media guide for HP printers.

http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/bpl05339/bpl05339.pdf

Print in grayscale (black area streaking only) If the streaks are in a black area of a black and white printout, change to grayscale printing.
NOTE: This step does not apply to color printouts.

1. In the application you are printing from, click File , and then click Print .
2. Make sure the printer is selected and click Properties .
3. Click the Color tab, and select Print Grayscale and High Quality .

Can't read the entire heading for your post, but it sounds like a defective toner cartridge. Replace it.