There are several GUI libs that were started but not maintained. The main ones that are maintained are Gtk2Hs and wxHaskell.
There is no current release of Gtk2Hs or wxHaskell for GHC 6.6 on Windows (that i know about), in my opinion that means they are not supported enough to actually use. That said, I'd use GHC 6.4.2 and Gtk2Hs, because it is supported. Also the first person to reply to your post was Duncan, who is one of the Gtk2Hs developers - I usually find that he provides excellent support and help where required.
I use wxHaskell with GHC-6.6. It was released pretty soon after GHC-6.6 was out. The trick is that it was released by Shelarcy here: https://sourceforge.net/project/showfiles.php?group_id=168626 I found it by Googling for this message: http://www.mail-archive.com/haskell-cafe@haskell.org/msg14839.html