Vista - Password works on network but not for login

I am running vista mce on a home network (wired). I have another pc running xp home edition which is networked to the vista pc. During the process of sharing drives for backup I created a login password on the vista machine and used the same login name and password on the xp pc to access the vista pc when requested. Both pc's on mshome network. Everything worked fine.

Today I tried to login to the vista pc and I'm told user name or password is incorrect. I am 100% certain it is correct. This is the first time I have tried to log in having set up password. There is only one user set up on the pc. With the vista pc switched on but stuck at the login screen I tried accessing it via the xp pc over the network. When asked for user name and password I entered the vista user name and password on the xp pc and was able to access the shared drive!! Unfortunately the shared drive doesn't have vista os installed on it. I have tried disconnecting the xp pc and taken it off network-still can't access vista with same details. I haven't created a password reset disc (never again). Started to look at using Trinity Rescue Kit but hoping someone here can show me the error of my ways!