Setting aside the fact that editing Comic Sans MS is almost certainly a technical breach of copyright, even if the amended version is only for your own use, you should be editing a copy of the file rather than the installed original. Which version of FontCreator do you have, and what version of Windows are you using?
You might find that Trebuchet MS is better suited to your needs. I have read that it has a good reputation among people with dyslexia.
Install that and see how you like it. You can modify that one, and share it without infringing any copyrights, though read the terms of the license first — there are still limits to what you can do with OpenSource fonts.
I have no problem copying fonts from the Fonts folder on Windows 7, and I’d be very surprised if that side of things has changed so much in Windows 8. If you right-click a font file there should be a ‘Copy’ option on the context menu, and you should be able to paste to any location where you normally save documents or downloads.
I still think you should consider alternatives to Comic Sans MS. I had forgotten about OpenDyslexic: why not follow up Bhikkhu Pesala’s suggestion?
As well as renaming the Truetype Font file in Windows Explorer, you also have to rename the Font Family name in Font Properties to something other than “Comic Sans MS”