Any fonts installed to your "C:\Windows\Fonts" folder will be available all the time, in every program that uses fonts. To install a font this way, use your Windows Explorer to browse to the "Fonts" folder, then click "File > Install New Font..." Typically, you can also just copy and paste a font file into your "Fonts" folder, and it achieves the same result.
There's a catch to that, however...
Windows can only handle a certain number of fonts -- typically around 1,000; usually slightly less. This is because every font you install to the "Fonts" folder creates an entry in your system registry file. Windows limits the size of the fonts portion of this file, which results in the limit of installed fonts.
To bypass this, you can keep the font files (.ttf or otherwise) in another location on your hard drive. When you want to use a font, double-click the file to open up the font preview window. The font will be temporarily loaded -- not installed -- so that you can use it in your current project. When you close the font's preview window, it's unloaded and again unavailable to use.
Personally, I prefer to max out my "Fonts" folder. I purchased a really nice font program -- Font Xplorer, which is now totally FREE -- a while back, and it's invaluable to me to be able to compare fonts against each other to see how someone's name will look, or to make sure that a font I like has a certain character mapped (typically, it's the problem of "demo" fonts not having ampersands or dashes).