Here are step-by-step instructions on how to install fonts onto your computer: (after downloading the zip file from www.dafont.com, or any other font site. Sometimes it might not be a zip file but in most cases it is.)
1. Right click on your zip file, and choose the option "Extract all".
2. Follow the steps on the program that will help you extract the files. Save the folder on your desktop.
3. Open the newly created folder that should appear on your desktop after the files are extracted.
4. In the folder should be a few files. One of the files should bear an icon: a piece of paper with a letter on it, O, A or double T's.
5. Copy that file.
6. Go to Start> Control Panel> Fonts, and paste. Alternatively, you can go to Start> My Computer> C drive> Windows> Fonts. But the first one is easier.
7. It's installed :D And you should be able to use it in many applications that allows you to customize fonts. Done!
If you get an error message saying that the file is in use, you could try installing this program
http://ccollomb.free.fr/unlocker/
called Unlocker. Whenever you get one of those error messages, Unlocker will help you to figure out which program is using that file, so that you can stop it. Install it then run it, and then try installing fonts again. If you get the error message, click OK. A second later Unlocker should come up showing which program it is. If you're not using it, you can terminate it.
Now, if my answer didn't help you, you can email me with more details and I'd be glad to help! :D