Q. Anything printing out of a certain printer is very dark and hard to read, but documents printed to another printer come out fine.
A. Print a Printer Test Page by going to Start / Settings / Printers or Start / Printers and Faxes. Right click on the printer you are working with and select Properties from the drop down list. This will display a window for the printer properties. Click on the Print Test Page button.
If the test page also prints out dark, then the problem is most likely with this printer. Updating to the most current version of the printer driver is one good suggestion to try first. If that doesn't work, try replacing the toner cartridge.