This happened to me today. When I viewed the font in the Finder, the font had no filename extension, but Get Info said its Kind was "Postcript Type 1 outline font." Unfortunately, when I zipped the file and opened the zip file on Windows, the file was 0KB long; not very useful!
Here's how I fixed the problem.
To convert it, I used FontForge. FontForge is a Unix utility that kinda sorta works on OSX. They don't provide an installer for OSX, so the easiest way to install it is by using MacPorts. MacPorts comes with an installer, but you have to install Xcode in order to use it. Xcode can be found on your original OSX install DVDs, on DVD 2. If you can't find those, you can download Xcode for free from Apple, but you have to register as an Apple developer. It's a hassle.
Anyway, assuming you can find and install a copy of Xcode and MacPorts, you should be able to run "sudo port install fontforge" and wait an hour for macports to download a bunch of code from the Internet, compile it, and install it on your machine.
Finally, finally, you can run "fontforge" to open the program. Double-click on your font, and select the File / Generate Fonts ... menu. Select "TrueType" in the list of font types, and save it. It might give you some warnings; I ignored all warnings with no ill effects, but your mileage may vary. This created a .ttf font which worked great on my Windows machine.