This is my thinking on what you should be checking for - done as a list to get things straight in my own head more than anything ;-)

The back-end

• In cmd.exe, chcp 65001 to set utf-8 code page (this will probably only last for as long as you have the same cmd.exe window open).
• Open up the .txt files in an utf-8 capable editor and verify that the contents look ok.
• Save the files using greek characters in the filename - I know that MS Word, for one, can do that, so you can always use that if needs be.
• Check that dir shows you ok-looking greek characters.
• If it does, proceed to the web side of things - if not, you'll have to figure out what the issue is before even attempting to get it sorted on your web pages.

The web

• Check that your web pages are saved as utf-8, and that they're being served up as utf-8 (Firefox will show you this in 'Tools -> Page info').
• Remove all the from_to code from your form
• Make sure that your browser/font settings are sane (i.e. if you're specifying a font, that it actually has greek characters in it) - I'm sure you are though
• ...
• Profit! ;-)

That's about all that I can think of right now, but that should get you most, if not all, of the way there.