I installed this font: http://www.dafont.com/dk-lemon-yellow-sun.font into my Downloads folder and extracted the file. Now whenever I open the Downloads folder my computer crashes and has to be restarted. I am led to believe it was a virus. I thought Dafont.com was a safe site for fonts?

After you extracted the file, what did you do?

I opened the downloads folder to have a look at the font and the computer crashed.

I cannot reproduce that error. Are you on a fixed connection or on WiFi? When on WiFi try to download again on a fixed connection, save the zip in your downloads folder, overwriting the existing zip file and then unzip again, overwriting the otf file.

WiFi at home. I can't save anything in the downloads folder, because whenever it is opened from any source the computer crashes. Even trying to save a picture, which annoyingly automatically opens up the download folder, makes the computer crash.

When you download (not install) to a folder that folder is not opened. The file is only saved in that folder.
Apart from all this, a font file cannot contain a virus. The checksum would be incorrect and therefore the font file can not be opened/installed.

As a rule of thumb it is always better to open a zip in an unzipper when downloading and then drag what you want to where you want it. Only save the zip when you need to.

Well, I've got no idea what's happened to the computer, but it definitely started after the file was extracted.

Extracted or unzipped?

Extracted