Nor could I find an IPv6 stack for Windows ME or 2000.
ME is in the 95/98 class. Win 2k on the other hand, I'm using IPv6 over it all the time and it works very well. If you're interested search for something called "IPv6 technology preview". It's still on MS servers somewhere (MSDN?) - alternatively, I think I took my copy from Sixxs or was it Go6/gogonet (I don't think I remember it was available from our here host HE/tunnelbroker, BICBW)
The MS stack is fully featured - as the name says it was a "preview" of what was to be delivered with Windows XP. The only significant missing thing is encryption (IPSEC /is/ present, but it can't encrypt. It may or may not matter to you...)