can only authenticate with ms-chap v1

My Server 2003 will only authenticate a vpn with MS-CHAP v1, every other
protocol returns that the username/password is not valid. This wasn't really
a problem with XP, some of my people now have Vista which doesn't allow v1.
I've checked/unchecked various different RAS protocol options on both ends
and it still ends up the same. I've also changed the remote access policies,
but with no luck. The RAS log for every protocol configuration except
MS-CHAP v1 shows that the username/password is invalid.
I'm sure that it's something on the server end, but I don't know where to
look.

Bill