Having an unencrypted vpn connection from a windows client to linux is pretty easy by using pptpd. When I was looking for an solution for encrypted (per MPPE) connection, I found a lot of information about patching the kernel etc. - so it definitly works after some work. But all these information is pretty old (2005-2006).
Is it the same solution nowadays? I am not asking for a complete instruction (only if it's short) - I am more asking for a link to the right solution.