Problem solved! The problem turned out to be the security setting of the computer.
Thanks for your help!

Thanks for doing that, I tried installing the file you uploaded and the same error message occurred. Could it be my computer's security settings that were preventing me from installing new fonts?

Because the same error message appears for all other fonts I downloaded here too.

Thanks for pointing that out!

I found the font here: Script > Handwritten Font name is: "Hand of Sean" by Nice and Ripe Ltd

So this is the message I've been getting: "The file Handsean.ttf does not appear to be a valid font."

I'm on Windows 7. After I unzipped the downloaded fonts folder, I tried to install the .ttf fonts in different ways: copy it into the fonts folder... right click and install... But the same message kept popping up: "The file.... does not appear to be a valid font."

I found the font here: Script > Handwritten Font name is: "Hand of Sean" by Nice and Ripe Ltd

Does anyone have the same problem? Thanks very much if you can help...