Check out
GTK-Wimp. It's a GTK theme (for GTK 2.x only, looks like) that passes the actual widget drawing through to the Win32 API, so all the GTK widgets look (more or less) like Windows ones.
Note, however, that I have no idea how you would go about installing it. The FAQ tells you how, but if you're using a copy of GTK installed with X-Chat rather than
the GTK runtime, it may not be much help.