Printers often alert you with low ink warnings prematurely in an effort to get you to buy replacements before you really need them. If your printer gives you a message that ink is low, you can often override the ink level warning to continue printing. Note that this will only work for so long though. Eventually you will actually be out of ink and will need to get a new cartridge.
Restart your printer. Make sure it is connected to your computer.
Open the Control Panel from the PC's Start menu. Select "Printer and Faxes."
Right-click your printer's icon (be sure to choose the right printer if you have used more than one with your computer). Select "Properties" from the list that pops up.
Go to the "Services" tab. Click "Service This Device."
Click the "Estimated Ink Level" tab. This will have been recalculated when you restarted your printer in Step 1 and will allow you to see exactly where your ink levels are.
If you do need to purchase new cartridges, you should always bring along the old ones. Not only is it easy to accidentally buy the wrong ink, but many stores offer discounts if you recycle your empty cartridge with them.