.. and what is "hide"? (in the Windows-Font right-click dialog box). The icon fades, but is still available in Word. So what am I hiding it from?
Windows 7 added the "hide" feature. The purpose is to hide an installed font so that it doesn't show up in application menus but will still be installed so that applications that are using them internally will not be impacted. You can imagine the problems you would have if you uninstalled the font your application (or Windows itself) was using for its menus and error messages.
The problem is that the fonts are hidden only if the application is using the new "choose font" routine or the new Scenic Ribbon controls that Windows 7 provides for the application's use. If the application is using its own custom routine to get the list of fonts to display and choose, it has no idea whether the font is hidden or not. So, it shows them. Applications using custom font menus will need to be rewritten to check the hidden status of the font.
Is there any way to know what fonts I can safely delete from my computer .. other than perhaps moving them to another folder to see if anything will go wrong .. go wrong .. go wrong
It's difficult. I haven't seen a list of required fonts for Windows 7. But, even if you don't delete a system font, your installed applications may be using a font for some purpose and you might not find out until long after you delete the font. The only fonts you know for certain won't cause problems when deleted are the ones you added yourself. I'd recommend always copying the font to a backup folder before deleting it.