This guide is for people who have a Linux computer and a Windows computer, and the printer is on the Linux computer.
1. Make sure that both computers are behind the same router.
2. Install the printer you want to share on your Windows computer if it hasn't been installed already.
3. In Windows click Start -> Settings -> Printers and Faxes (or Start -> Printers and Faxes with the XP-style start menu) and open the properties for the printer you wish to share. Go to the sharing tab and select Share this Printer. Give it a share name that you can remember with no spaces.
4. In Linux go to Start Application -> System Settings -> Printing (in KDE). Choose to install a new printer. Go forward until you get to the Queue type page. On that page select Networked Windows (SMB) at the top, and then click specify on the right. Enter your Windows computer's workgroup (eg. MSHOUSE), computer name (You probably entered this when you installed Windows), your printer's share name from before, and your user name or password on the Windows computer. You can usually get by if you but guest in the user name box and leave the password box blank. Click OK. From there choose your printer's drivers. Linux has many drivers pre-installed so you shouldn't have to go to your printer manufacturer's website to download them. Once you're done, print a test-page to see if it worked.
5. Give yourself a pat on the back because you have accomplished the impossible.